Stories revolving around alien life and UFO incidents are nothing but mere mystery. And when Chinese astronauts kicked off their space missions, they heard puzzling knocking sounds in space.

The Chinese space agency has geared up to travel and explore the hidden side of space. Yang Liwei, the first Chinese astronaut, who travelled to space back in 2003 experienced a bizarre case that led him to believe that the space is filled with obnoxious entities.

According to the astronaut, his first flight experience was jam-packed with mystery as he heard knocking and banging sounds in space.

This was unveiled during a recent interview when Yang Liwei said that the sound resembled a hammer hitting a metal surface. During his first mission, the astronaut claims that the sound was breaking off in the middle, continually appearing and disappearing in between.

"A non-causal situation I have met in space is a knock that appeared from time to time," Yang Liwei said in a TV guesting in China according to IB Times. "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 Liwei added.

 Yang Liwei's statement was supported by another Chinese astronaut who accompanied him during the mission. The now senior general and former astronaut also tried to analyze the source of the knocking sound, but was not fruitful.

The sound disappeared when the astronauts returned to the land. But the mystery did not end there. The astronauts who went to the space again aboard the Shenzhou 6 and Shenzhou 7 following Yang Liwei's mission also claimed to have heard the knocking and banging sounds.

"Before entering space, I have told them that the sound is a normal phenomenon, so there is no need to worry," said Liwei in the same report.

According to Express, the odd and unidentified nature of the metallic sounds might indicate the presence of extra-terrestrial beings. But scientific theories suggest that the sounds might be originating from a part of the spacecraft constantly hitting each other.

However, no one is really sure regarding the origin of the sounds. Is this proof that there is alien life? As far as the research is concerned, Yang Liwei has not yet been able to reach a determined conclusion.