An investigation into underwater noises coming from the area of the search for the missing Titan submersible have come up "negative", officials have said.

Regular banging sounds were picked up by a Canadian plane with underwater sonar capabilities after it was deployed in the search for the missing vessel.

The banging sounds were detected every 30 minutes on Tuesday - and were picked up again when search teams deployed more radars in the area.

But the US Coast Guard said on Wednesday an investigation of the noises by specialist underwater equipment had "yielded negative results".

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"Additionally, the data from the aircraft has been shared with our US Navy experts for further analysis, which will be considered in future search plans," the US Coast Guard added in a post on Twitter.

The noises were initially picked up by a Canadian P-3 aircraft using sonobuoys - a special type of buoy with underwater sonar capabilities.

What is a sonobuoy?

Used in everything from research to rescue missions and even warfare - sonobuoys are an important tool for underwater searches.

The devices are dropped from aircraft into the water, where they descend to the required depth.

A surface float with a radio transmitter is also dropped to maintain communication between the sonar and the aircraft above.

The sonars emit sound energy - commonly referred to as a "ping" - and then wait for the returning echo to map the water and locate any objects in it.

Once the equipment picks up the echo, it transmits the information back to the surface buoy and then back to the aircraft. These are known as "active" sonobuoys.

There are also "passive" sonobuoys, which are used to simply listen for sounds in the water - such as the sound of enemy submarines.

The buoys were initially developed in the Second World War to detect German U-boats - and remain an important tool in underwater warfare.

But the devices are now deployed for a variety of other purposes, including in search and rescue operations, where they can map the location of an aircraft crash site, a sunken ship, or survivors at sea.

Five vessels are now at the Titanic wreck site, with four others listed by the US Coast Guard also on their way.

However, they will not get there today, according to marine tracking data.

The research vessel Polar Prince - which towed the Titan submersible - is still at the wreck site, alongside the Bahamian pipe laying vessel Deep Energy, which joined yesterday morning.

Commercial Vessel Skandi Vinland arrived around 7pm local time on Tuesday, while Canadian coast guard vessels John Cabot and Atlantic Merlin arrived overnight.

The French research vessel L'Atalante is likely to arrive this evening.

Image: A map showing the ships already at or travelling to the Titanic wreck site.

It comes after a former employee of the missing Titan submersible operator has revealed he had raised "safety concerns" over the vessel but was reportedly "met with hostility" before later being sacked, according to court documents.

OceanGate's former director of marine operations, David Lochridge, had raised concerns over "safety and quality control issues regarding the Titan to OceanGate executive management", according to the filings.

In the August 2018 court document, it claims chief executive and founder of OceanGate Expeditions Stockton Rush, asked Mr Lochridge to conduct a "quality inspection" report on the vessel following the "issues of quality control".

Mr Lochridge "identified numerous issues that posed serious safety concerns" but he was reportedly "met with hostility and denial of access" to necessary documents.

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How the missing sub saga unfolded.

The court filing claims he was worried about a "lack of non-destructive testing performed on the hull of the Titan", and that he "stressed the potential danger to passengers of the Titan as the submersible reached extreme depths".

Mr Lochridge was later fired from the company, wrongfully he claims.

OceanGate's icebreaker, Polar Prince, which was supporting Titan, reportedly lost contact with the vessel about an hour and 45 minutes after it submerged.

The five men on board are Stockton Rush, British billionaire Hamish Harding, Pakistani businessman Shahzada Dawood and his son Suleman, and French submersible pilot Paul-Henri Nargeolet.

Image: (Clockwise from top left) Paul-Henri Nargeolet, Stockton Rush, Hamish Harding, Suleman Dawood and Shahzada Dawood

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They had "about 40 hours of breathable air" left, the US Coast Guard said on Tuesday night.

A race-against-time search and rescue operation is taking place some 435 miles south of Newfoundland, Canada.

The wreckage of the Titanic, which sank in 1912 after hitting an iceberg, lies at a depth of about 12,500 feet (3,810 metres).

The Titan submersible usually takes two hours to descend to the wreck.