Played for the first time in a new documentary, the sound has a regular, steady beat which caught the attention of experts - and sparked hopes that they could be SOS signals from those on board.
It had been thought the banging was SOS noises by the five men. But we now know the sub was crushed as soon as it encountered trouble - so what was the banging. MailOnline investigates.
As the oxygen levels continue to decrease by the minute, time is running out to save the five on board the Titan submarine, but an expert has explained hope is still alive.