The Sound That Has Never Been Explained

From Yahoo!: "A Chinese Astronaut Heard A Sound In Space That Hasn't Been Explained In Two Decades." An excerpt:

    It was October 2003 and a historical moment for China. The country launched its first manned spacecraft carrying astronaut Yang Liwei, and his flight was scheduled to be 21 hours long. During his flight, however, he heard a knocking sound. He described the sound like a wooden mallet hitting a metal surface. The sound occurred on and off. He could not identify the source of it, though reported that it sounded like something was knocking on the outside of the ship.

    Adding to the mystery, this was not the last time the sound was heard. Astronauts on Shenzhou 6 in 2005 and Shenzhou 7 in 2008 also reported hearing the exact same sound and were equally unable to identify the cause. Though not exactly the same sound, during the Apollo 10 mission in 1969, astronauts reported hearing eerie whistling while they were on the dark side of the moon; the whistling was even recorded by the ship's systems. Most recently, the astronauts Butch Wilmore and Sunita Williams, famously trapped in space after the fiasco with their Boeing Starliner spacecraft, heard a strange pulsing noise from the ship.

    In many of those cases, scientists could find a source for the strange sounds. The Boeing Starliner noise was explained as audio feedback from the Starliner and the International Space Station. The Apollo 10 noise was explained as radio interference between the lunar module and the command module. However, the knocking sounds heard on the three Shenzhou missions have still eluded explanation.

    The main reason that sounds in space are alarming is because sound waves have to be able to bounce off of objects to create noise. National University of Singapore professor Goh Cher Hiang explained this to the BBC: "The travelling of sound travels requires a medium — be it air particles or water molecules or metal, solid atoms ... If it is knocking, there could be something physical 'hitting' the spacecraft carrying the astronaut."   

Whatever it was, it probably had a bloody steel hook for a hand.