Chinese scientist hears 'knocking sound' in space

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

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