"A non-causal situation I have met in space is a knock that appeared from time to time." he said during an interview. The Liaoning Province native claimed that the sound came neither “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." According to Xinhua News Agency.

He explained that the noises would come in an instant and seemingly without cause. Though he saw nothing inside or outside of the spacecraft’s porthole, the sound still made him nervous.

Yang informed technicians of the unexplained noise once he returned to Earth, and tried using instruments to reproduce the sound, but has not heard a sound quite like it since he landed.

Astronauts aboard the Shenzhou 6 and Shenzhou 7 have also heard the noise, but the 51-year-old said he has reassured them that they aren’t in danger. "Before entering space, I have told them that the sound is a normal phenomenon, so there is no need to worry."