Missing Submarine Search Crews Hear 'Banging' Noises On Their Sonars

Search crews desperately looking for the five people trapped inside a submarine that vanished in the Atlantic Ocean might have gotten their first break -- after hearing possible signs of life coming from the vessel.

According to CNN, citing an internal U.S. government memo, the crews detected banging noises in 30-minute intervals over their sonars emanating from inside the North Atlantic Ocean Tuesday after the submarine disappeared two days earlier while on a tourist exploration to see the Titanic wreckage.

The internal memo said additional sonar devices were also deployed Tuesday, picking up even more banging, although it was unclear how long the second batch of sounds lasted.

"Additional acoustic feedback was heard and will assist in vectoring surface assets and also indicating continued hope of survivors," CNN reported, quoting the internal document.

What's more ... a Canadian P-3 military aircraft reportedly spotted a white rectangular object in the ocean, but the potential lead has yet to be investigated because crews were too busy probing the banging noises.

The U.S. Coast Guard tweeted ... the P-3 aircraft shared information about the rectangular object with the U.S. Navy "for further analysis which will be considered in future search plans."

As everyone knows by now ... the authorities are in a race against time to find the submarine and save its five passengers, who began with 96 hours of oxygen when they launched the voyage Sunday.

By Tuesday, that number was cut by more than half to 40 hours. Now it's Wednesday and the clock is still ticking.

Former Coast Guard Captain Andrew Norris told us on "TMZ Live" ... the submarine passengers would need an unprecedented miracle to survive this situation.