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China's inaugural astronaut returned to Earth with an extraordinary account after encountering a puzzling 'knocking sound' that left space engineers completely stumped.

Taikonaut Yang Liwei launched into orbit aboard Shenzhou 5 in 2003 for a 21-hour mission that saw him circle the globe 14 times before touching down safely in Mongolia.

However, during his journey through space, Yang detected something utterly inexplicable: a distinct 'knocking' sound repeating itself continuously. He examined the capsule thoroughly, gazed through the porthole, and inspected the panels, yet couldn't identify the source, even though the sound persisted.

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He maintained it wasn't originating from inside the capsule, but rather from outside, in the vacuum of space, where sound, theoretically, cannot travel.

Yang subsequently revealed to the BBC: "It sounded like someone hitting the hull, like striking an iron bucket with a wooden hammer."

He remained composed and continued his mission, ultimately dismissing the unsettling tapping as a 'normal phenomenon'.

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Upon returning to Earth, engineers meticulously examined the spacecraft for answers but discovered nothing that could explain the noise, reports the Daily Star.

Yet the peculiar sound didn't disappear with Yang, as Chinese media subsequently reported that crews on later missions - including voyages in 2005 and 2008 - also experienced the same mysterious knock. Prior to launch, Yang even warned newcomers not to be alarmed if space appeared to come knocking at the door.

Sound requires a medium, such as air or metal, to travel, so a knock from the vacuum makes no sense.

Potential explanations put forward included tiny micro-debris striking the hull without leaving visible marks. While plausible, no evidence was discovered during Yang's mission.

Alternative theories included thermal fractures as the spacecraft experiences extreme temperature fluctuations every 90 minutes in orbit, and structural or mechanical vibrations triggered by pressure systems, valves, reaction wheels or attitude thrusters.

China's more recent spacecraft are equipped with additional sensors, and every unusual creak and tremor gets recorded. However, until the data provides a conclusive answer, Yang's 'knock-knock' remains amongst spaceflight's most intriguing unsolved mysteries.