China’s first astronaut claims he heard strange knocking sounds while he was in space in 2003. According to 51-year-old astronaut Yang Liwei, the noise he heard aboard the Shenzhen 5 spaceship sounded like a hammer hitting an iron bucket.

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

Liwei felt very nervous upon hearing the knocking noise. Apparently, he went close to the spaceship’s porthole to find out what it was but he did not see anything.

Liwei initially refused to talk about the knocking sounds because he thought it was nothing out of the ordinary. However, it was revealed that other astronauts aboard the Shenzhou 6 and Shenzhou 7 also heard the same sounds.

Still, the astronaut explains that the sounds were probably a normal phenomenon. Despite this conclusion, the 51-year-old admits that the sounds still haunt him.

The online magazine Inverse tried to find out more about this phenomenon. The website asked NASA if American astronauts also experienced the same incident.

“As far as we know, there haven’t been any reports of such sounds,” NASA’s Kathryn Hambleton told Inverse.

Inverse states that it was probably caused by something that hit the spaceship. According to Wee-Seng Soh, an expert in space engineering at the National University of Singapore, the sounds could have also been the result of the spaceship’s expansion or contraction.

Wee-Seng Soh told the BBC that this is a probable explanation since the temperature of a spaceship’s exterior fluctuates while in space. It was pointed out that China is still relatively new to manned space missions so they were not probably used to such occurrence.