The sounds were picked up by a Canadian plane using buoys with underwater sonar capabilities while searching for the missing vessel.

However, the US Coast Guard said on Wednesday that an investigation of the noises using specialist underwater equipment had so far "yielded negative results".

It comes after US media outlets, citing emails from the US Department of Homeland Security, reported there were "banging sounds in the area every 30 minutes" on Tuesday.

Titanic sub search - live: Banging picked up by sonar devices

Searches continue for the vessel and at least five ships are in the area - with four more thought to be on the way.

But search teams face a frantic race against time to find the submersible - and rescue its five occupants - with experts suggesting that if the vessel is still intact, it would likely have less than 24 hours of oxygen left.

Noises a 'great sign' but could raise 'false hope'

Dr Jamie Pringle, a reader in forensic geosciences at Keele University, said the noises are good news but warned it may give "false hope".

He said the fact the noises are half an hour apart is a "great sign" and means they are "unlikely" to be from another submarine or a surface ship propeller.

"Acoustic noise travels far in water so that is both good and bad news - you would still need three of those static buoys to be able to triangulate the sound source to get a position fix."

Dr Keele said the use of sonobuoys - buoys with sonar capabilities - differs from a sonar survey, which uses either a towfish from a surface vessel or an unmanned self-propelled one for deeper waters.

"The sound could, of course, be coming from something other than the sub," he said. "Let's not give people false hope here, the lack of oxygen is key now - even if they find it, they still need to get to the surface and unbolt it."

He also added: "It might be nice and flat and it be sitting proudly on top, which would be great. It could be partially submerged, and of course, there's lots of submarine canyons and submarine seamounts... those issues are very difficult."

Banging sounds could be from Titanic itself

The banging sounds could be "spurious" as there is "a lot of noise coming from the ocean", an ex-Navy officer has said.

Former rear admiral Chris Parry said the Titanic itself "makes a lot of noise even as the current shifts through it", and attention should be focused on the area of "likely accident to the vessel itself".

Analysing the current situation with the now-international rescue effort for the submersible, Mr Parry said there is a "concentrated section of aircraft and support vessels" around the Titanic wreck site.

Titan is a "very small target" and a remote operating vehicle from Canadian offshore vessel Skandi Vinland did not turn up anything after being deployed on Tuesday.

But "over the next few hours, I think we're going to see a significant uplift in capability," Mr Parry added. "The assets are coming together."

'A hostile environment'

Dick Barton, former vice president of operations with RMS Titanic Inc, the company which owns the salvage rights to the Titanic, said it was important to "not get too ambitious".

"Tapping is a well known means of emergency communication... but I think it needs to be kept in context because the ocean is a very busy place," he told Sky News.

"There's lots of activity down there - marine life, submarine and underwater cabling and in general lots of activity. So let's not get too ambitious here."

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Mr Barton, who was the first Briton to complete a dive to the Titanic wreck, added: "It is a pretty hostile environment [down there].

"It is two-and-a-half miles below the surface. It is cold, there is zero light. Basically, you are on your own."

'Noise gives hope this is still a rescue operation'

Noises heard underwater on Tuesday provide hope that the crew on board Titan are still alive, according to Dr Simon Boxall, a senior lecturer in oceanography at the University of Southampton.

"There are plenty of sound sources in the ocean, but it does give hope," he told the BBC's World Service.

Dr Boxall said one of the feared scenarios was that the submersible had imploded due to the pressure.

"It does give some hope that this is still a rescue operation, rather than just a recovery operation," he said.

What happens if they find the Titan?

While finding the submersible remains priority number one, carrying out a rescue will also prove a significant challenge - all against the ticking clock of a dwindling air supply.

According to marine operations specialist and author Mike Welham, simply locating the vessel is "like somebody putting a 50p piece in a football field and trying to find it".

Mr Welham also pointed out that procuring the specialist equipment and manpower for an effective rescue at the depths Titan can reach "takes time".

He said those aboard the Titan were at a sea level where they would be "crushed" by the water pressure - meaning the only way to save them would be to bring the vessel to the surface.

"The circumstances are against it," he said, but added that the bangs heard in 30-minute intervals suggested that someone had "taken charge" of the submersible and was "sending out the taps".