A Chinese astronaut has reported hearing a strange “knocking” sound in space – and no one is quite sure what it is.

The phenomenon was brought to light by Yang Liwei, who became China’s first astronaut (known as a “taikonaut”) on October 16, 2003. He flew to space on board the Shenzhou 5 spacecraft, and made China the third country – after the US and Russia – to independently send an astronaut to space.

But speaking recently to China Central Television, he reported hearing something strange on that first 21-hour mission.

“A non-causal situation I have met in space is a knock that appeared from time to time,” he said, reported Xinhua.

“It neither came from outside nor inside the spaceship, but sounded like someone is knocking the body of the spaceship just as knocking an iron bucket with a wooden hammer.”

He added that he was nervous when he heard the sound, and looked out the porthole to try and see the cause, but saw nothing of note. Interestingly, the same sound was heard by other Chinese astronauts on the Shenzhou 6 and 7 spacecraft in 2005 and 2008, respectively.

There are several possibilities for what the noise might have been. The first is perhaps space debris striking the spacecraft, although this seems somewhat unlikely. Perhaps more plausibly, it may have been the result of the spacecraft expanding and contracting in the low-temperature environment of space.

This isn’t the first time strange sounds have been heard in space. The Apollo 10 crew, on their journey around the Moon in May 1969, famously heard an unexplained whistling sound, which was likely some sort of radio interference.

It’s enough to get your tin foil hat bristling, but rest assured – there’s probably some rational explanation, rather than some rogue aliens trying to make contact. The knocking sound is still pretty weird though, and the fact that it only seems to have been heard on these Chinese spacecraft suggests it might be a design issue that’s common across the Shenzhou vehicles.

Fortunately, all of those missions were carried out successfully, so whatever it was, it doesn’t seem to have posed any danger to the crews.