An upcoming documentary on the deadly Titan submersible implosion includes audio of mysterious knocking sounds which raised hopes that passengers were still alive during the search for the vessel last summer.

The strange noises, heard in a clip shared by LADbible of ITN Productions documentary for the U.K.s Channel 5, were picked up as rescuers searched for the missing sub that sought to visit the Titanic wreck in June.

Sounds like it could be somebody knocking, the symmetry between those knockings is very unusual, Ryan Ramsey, a former Navy submarine captain in the U.K., says in the clip.

Its rhythmic, its like somebody is making that sound and the fact that its repeated is really unusual.

The newly-released audio of the sounds was picked up by the Canadian Air Force during the search for the vessel.

Its expected to air on Channel 5 as part of a two-part documentary, The Titan Sub Disaster: Minute by Minute, on March 6 and 7.

The disaster killed five people including Stockton Rush, CEO and co-founder of the company behind the sub, OceanGate.

The sounds were initially reported amid the search and were described as banging noises heard at roughly 30-minute intervals, according to CBS News .

A U.S. Navy analysis found the sounds were either ocean noise or stemmed from other search ships, an official said.

A senior military official would reveal that a U.S. Navy acoustic system detected an anomaly, what was likely the implosion on June 18 the date of the vessels descent.

Experts noted that an implosion wouldve destroyed the Titan nearly instantaneously giving the subs passengers no opportunity to signal for help, CBS News reported.