Chinese scientist hears 'knocking sound' in space

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In space, no one can hear you scream -- but you may hear a knock.

When he was alone in a spacecraft in 2003, astronaut Yang Liwei reportedly heard a "knock" despite being alone. Liwei was the first man China sent to space, so there's no question that he was by himself.

In a recent interview, the astronaut said that the sound was like "someone knocking the body of the spaceship just as knocking an iron bucket with a wooden hammer."

See fascinating photos of Jupiter

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NASA's Juno spacecraft lands on Jupiter
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NASA's Juno spacecraft lands on Jupiter
An artist's rendering depicts NASA's Juno spacecraft above Jupiter's north pole in this undated handout image. Launched in 2011, the Juno spacecraft will arrive at Jupiter in 2016 to study the giant planet from an elliptical, polar orbit. Juno will repeatedly dive between the planet and its intense belts of charged particle radiation. NASA/JPL-Caltech/Handout via Reuters ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY
Members of the Juno team celebrate at a press conference after they received confirmation from the Juno spacecraft that it had completed the engine burn and successfully entered into orbit around Jupiter,at the Jet Propulsion Laboratory in Pasadena, California, U.S. in this July 4, 2016 handout photo. The Juno mission launched August 5, 2011 and will orbit the planet for 20 months to collect data on the planetary core, map the magnetic field, and measure the amount of water and ammonia in the atmosphere. NASA/Aubrey Gemignani/Handout via Reuters ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY
(L-R) Dr. Jim Green, Planetary Science Division Director, NASA; Scott Bolton, Juno principal investigator, Southwest Research Institute; Geoff Yoder, acting Associate Administrator for the Science Mission Directorate, NASA; Michael Watkins, director, NASA?s Jet Propulsion Laboratory (JPL); and Rick Nybakken, Juno project manager, Jet Propulsion Laboratory (JPL); celebrate with others on the Juno team after they received confirmation from the spacecraft that it had successfully completed the engine burn and entered orbit of Jupiter, in mission control of the Space Flight Operations Facility at the Jet Propulsion Laboratory in Pasadena, California, U.S. in this July 4, 2016 handout photo. The Juno mission launched August 5, 2011 and will orbit the planet for 20 months to collect data on the planetary core, map the magnetic field, and measure the amount of water and ammonia in the atmosphere. NASA/Aubrey Gemignani/Handout via Reuters ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY
A 1/4 scale model of NASA's Juno Spacecraft is seen in front of an image of Jupiter, at the Jet Propulsion Laboratory (JPL) in Pasadena, California, July 3, 2016. NASA's solar-powered Juno spacecraft is scheduled to enter into orbit around Jupiter on July 4 to begin an in-depth study of the planet's formation, evolution and structure. The key event on July 4 is a 35-minute engine burn at 11:18 p.m. EDT (0318 GMT on Tuesday), which is designed to slow Juno down enough to be captured by Jupiter's powerful gravity. / AFP / Robyn Beck (Photo credit should read ROBYN BECK/AFP/Getty Images)
NASA's Juno Mission Principal Investigator Scott Bolton (L) and Robert Kondrk (R), Apple vice president for Content and Media Apps, speak at a press conference at the Jet Propulsion Laboratory in Pasadena, California, June 30, 2016 to announce 'Destination: Juno,' a collaboration between NASA and Apple to bring 'exploratory' music inspired by space from artists such as Brad Paisley, Corinne Bailey Rae, GZA, Jim James featuring Lydia Tyrell, Trent Reznor & Atticus Ross, Weezer and Zoé to Apple Music and iTunes listeners. The Juno spacecraft is scheduled to enter Jupiter's orbit on July 4, 2016 after a five years voyage to the fifth planet from the sun. / AFP / Robyn Beck (Photo credit should read ROBYN BECK/AFP/Getty Images)
NASA Program Executive Diane Brown (L), Juno Mission Principal Investigator Scott Bolton (C) and Robert Kondrk (R), Apple vice president for Content and Media Apps, attend a press conference at the Jet Propulsion Laboratory (JPL) in Pasadena, California, June 30, 2016 to announce 'Destination: Juno,' a collaboration between NASA and Apple to bring 'exploratory' music inspired by space from artists such as Brad Paisley, Corinne Bailey Rae, GZA, Jim James featuring Lydia Tyrell, Trent Reznor & Atticus Ross, Weezer and Zoé to Apple Music and iTunes listeners. The Juno spacecraft is scheduled to enter Jupiter's orbit on July 4, 2016 after a five years voyage to the fifth planet from the sun. / AFP / Robyn Beck (Photo credit should read ROBYN BECK/AFP/Getty Images)
PASADENA, CA - JUNE 30: A scientist works at the Deep Space Network desk in the mission control room of the JPL Space Flight Operations Facility at JPL as NASA officials and the public look forward to the Independence Day arrival of the the Juno spacecraft to Jupiter, at JPL on June 30, 2016 in Pasadena, California. After having traveling nearly 1.8 billion miles over the past five years, the NASA Juno spacecraft will arrival to Jupiter on the Fourth of July to go enter orbit and gather data to study the enigmas beneath the cloud tops of Jupiter. The risky $1.1 billion mission will fail if it does not enter orbit on the first try and overshoots the planet. (Photo by David McNew/Getty Images)
PASADENA, CA - JUNE 30: Cassini Ace Bill Mogensen works at his desk in the mission control room of the JPL Space Flight Operations Facility at JPL as NASA officials and the public look forward to the Independence Day arrival of the the Juno spacecraft to Jupiter, at JPL on June 30, 2016 in Pasadena, California. After having traveling nearly 1.8 billion miles over the past five years, the NASA Juno spacecraft will arrival to Jupiter on the Fourth of July to go enter orbit and gather data to study the enigmas beneath the cloud tops of Jupiter. The risky $1.1 billion mission will fail if it does not enter orbit on the first try and overshoots the planet. (Photo by David McNew/Getty Images)
TOPSHOT - (From R) Rick Nybakken, Juno project manager, Scott Bolton, NASA principal investigator for the Juno mission to Jupiter and Jim Green, NASA director of Planetary Science, react as the Juno spacecraft successfully enters Jupiter's orbit on July 4, 2016, at the Jet Propulsion Laboratory in Pasadena, California. Juno was launched from Cape Canaveral in Florida on August 5, 2011 on a five-year voyage to its mission to study the planet's formation, evolution and structure. / AFP / POOL / Ringo Chiu (Photo credit should read RINGO CHIU/AFP/Getty Images)
Scott Bolton (L), NASA principal investigator for the Juno mission to Jupiter, reacts as the Juno spacecraft successfully enters Jupiter's orbit on July 4, 2016, at the Jet Propulsion Laboratory in Pasadena, California. Juno was launched from Cape Canaveral in Florida on August 5, 2011 on a five-year voyage to its mission to study the planet's formation, evolution and structure. / AFP / POOL / Ringo Chiu (Photo credit should read RINGO CHIU/AFP/Getty Images)
TOPSHOT - Juno Project Manager Rick Nybakken (C) celebrates as the solar-powered Juno spacecraft goes into orbit around Jupiter, at NASA's Jet Propulsion Laboratory in Pasadena, California on July 4, 2016. Juno was launched from Cape Canaveral in Florida on August 5, 2011 on a five-year voyage to its mission to study the planet's formation, evolution and structure. / AFP / POOL / Ringo Chiu (Photo credit should read RINGO CHIU/AFP/Getty Images)
Staff members watch on before the solar-powered Juno spacecraft went into orbit around Jupiter, at NASA's Jet Propulsion Laboratory in Pasadena, California on July 4, 2016. Juno was launched from Cape Canaveral in Florida on August 5, 2011 on a five-year voyage to its mission to study the planet's formation, evolution and structure. / AFP / POOL / Ringo Chiu (Photo credit should read RINGO CHIU/AFP/Getty Images)
Diane Brown (L), NASA Juno program executive, Scott Bolton (C), Juno principal investigator and Rick Nybakken, Juno project manager, celebrate at a press conference after the Juno spacecraft was successfully placed into Jupiter's orbit, at the Jet Propulsion Laboratory in Pasadena, California on July 4, 2016. Juno was launched from Cape Canaveral in Florida on August 5, 2011 on a five-year voyage to its mission to study the planet's formation, evolution and structure. / AFP / Robyn BECK (Photo credit should read ROBYN BECK/AFP/Getty Images)
Juno Project Manager Rick Nybakken (L) and principal investigator Scott Bolton (R) celebrate as the solar-powered Juno spacecraft goes into orbit around Jupiter, at NASA's Jet Propulsion Laboratory in Pasadena, California on July 4, 2016. Juno was launched from Cape Canaveral in Florida on August 5, 2011 on a five-year voyage to its mission to study the planet's formation, evolution and structure. / AFP / POOL / Ringo Chiu (Photo credit should read RINGO CHIU/AFP/Getty Images)
PASADENA, CA - JULY 4: Juno team members celebrate in mission control of the Space Flight Operations Facility at the Jet Propulsion Laboratory after receiving confirmation from the spacecraft that it has successfully entered orbit of Jupiter, July 4, 2016 in Pasadena, CA. The Juno mission launched August 5, 2011 and will orbit the planet for 20 months to collect data on the planetary core, map the magnetic field, and measure the amount of water and ammonia in the atmosphere. (Photo by Aubrey Gemignani/NASA via Getty Images)
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Understandably, the sound disturbed Liwei. He peeked around to try to find the source, but he could not -- and he still doesn't know where it came from. He also failed to recreate the sound to explain what he heard to other scientists.

There is a notion that space is completely silent, for there is no medium for sound to travel. The BBC reports, however, that there are "sounds of space." Other astronauts have had similar experiences of hearing unexplainable sounds. This whistling sound is an example -- but NASA says it's radio interference and not something extraterrestrial.

What was the knock that Liwei heard? We may never know, but that doesn't mean scientists will stop trying to find out.

RELATED: Astronauts return from space

24 PHOTOS
NASA's Soyuz spacecraft returns to earth
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NASA's Soyuz spacecraft returns to earth
A Russian Soyuz MS space capsule carrying International Space Station (ISS) crew members, Kate Rubins of the U.S., Anatoly Ivanishin of Russia and Takuya Onishi of Japan, descends near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool TPX IMAGES OF THE DAY
A Russian Soyuz MS space capsule carrying International Space Station (ISS) crew members, Kate Rubins of the U.S., Anatoly Ivanishin of Russia and Takuya Onishi of Japan, descends outside the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
A Russian Soyuz MS space capsule carrying International Space Station (ISS) crew members Kate Rubins of the U.S., Anatoly Ivanishin of Russia and Takuya Onishi of Japan, lands outside the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
A Russian Soyuz MS space capsule stands on the ground shortly after its landing with International Space Station (ISS) crew members Kate Rubins of the U.S., Anatoly Ivanishin of Russia and Takuya Onishi of Japan, as a rescue helicopter lands nearby, outside the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
A Russian Soyuz MS space capsule stands on the ground shortly after its landing with International Space Station (ISS) crew members Kate Rubins of the U.S., Anatoly Ivanishin of Russia and Takuya Onishi of Japan, as a rescue helicopter lands nearby, outside the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Specialists stand around the Russian Soyuz MS space capsule after its landing southeast of the town of Dzhezkazgan, Kazakhstan, Sunday, Oct. 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Russian space agency rescue team helps International Space Station (ISS) crew member Kate Rubins of the U.S. to get from the capsule shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Russian space agency rescue team helps International Space Station (ISS) crew member Anatoly Ivanishin of Russia to get from the capsule shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Russian space agency rescue team helps International Space Station (ISS) crew member Takuya Onishi of Japan to get from the capsule shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Specialists help International Space Station (ISS) crew member Takuya Onishi of Japan shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Russian space agency rescue team helps International Space Station (ISS) crew member Kate Rubins of the U.S. to get from the capsule shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
International Space Station (ISS) crew member Takuya Onishi of Japan waves shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Russian space agency rescue team helps International Space Station (ISS) crew member Takuya Onishi of Japan to get from the capsule shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Specialists help International Space Station (ISS) crew member Takuya Onishi of Japan shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
International Space Station (ISS) crew member Kate Rubins of the U.S. calls his relatives shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
International Space Station (ISS) crew member Takuya Onishi of Japan calls his relatives shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
The International Space Station (ISS) crew members Kate Rubins of the U.S., Anatoly Ivanishin of Russia and Takuya Onishi of Japan, surrounded by ground personnel, rest shortly after landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Russian space agency rescue team members carry International Space Station (ISS) crew member Anatoly Ivanishin of Russia shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Specialists help International Space Station (ISS) crew member Kate Rubins of the U.S. shortly after landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Specialists help International Space Station (ISS) crew member Kate Rubins of the U.S. shortly after landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Specialists help International Space Station (ISS) crew member Anatoly Ivanishin of Russia shortly after landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Russian space agency rescue team members carry International Space Station (ISS) crew member Takuya Onishi of Japan shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
Rescue team members carry International Space Station (ISS) crew member Kate Rubins of the U.S. shortly after the landing of the Russian Soyuz MS space capsule near the town of Dzhezkazgan (Zhezkazgan), Kazakhstan, October 30, 2016. REUTERS/Dmitri Lovetsky/Pool
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