Chinese astronaut Yang Liwei sits in the module of Shenzhen 5 spaceship. [Photo/Xinhua]

Yang, born in June 1965 in northeastern Liaoning province, is China's first astronaut and now a major general. On Oct 16, 2003, he stepped out of his re-entry module after a 21-hour flying in space, making him the 241st human being worldwide to visit outer space.

When he recalled his experience in space during a recent interview, he said, "A non-causal situation I have met in space is a knock that appeared from time to time."

He explained that the sound suddenly appeared without any rhyme or reason. "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," Yang said.

Yang said he was very nervous when he heard the sound, and moved close to the porthole to try to find out the cause. However, nothing out of the ordinary appeared both inside and outside.

After returning to earth, he told technicians about the mysterious sound, and tried to imitate it with some instruments, so as to solve the mystery. But Yang said he had never heard the exact sound again.

The sound has also been heard by Shenzhou 6 and Shenzhou 7 astronauts. Yang said, "Before entering space, I have told them that the sound is a normal phenomenon, so there is no need to worry."