It is unclear what some noises heard near the wreck of the Titanic actually are, the US Coast Guard has said.
As the search for the missing Titan submersible continues, Captain Jamie Frederick said: "We don't know what (the noises) are, to be frank with you."
The sounds were heard by a Canadian plane this morning and yesterday, Captain Frederick added.
"We're searching in the area where the noises were detected," he said.
The search area in a remote part of the Atlantic Ocean has been expanded and is now more than twice the size of the US state of Connecticut.
Captain Frederick said they "need to have hope" and that additional remote search devices with extra capabilities will arrive tomorrow.
A French submersible robot that can dive to 6,000 metres may be there late on Wednesday.
"Our efforts are solely focused on search," Captain Frederick said, adding: "This is a search and rescue operation - 100%."
Latest updates on missing sub
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The Victor 6000 robot is expected to arrive at the scene late on Wednesday
Former Royal Navy commander Chris Parry told Sky News he finds stories of "banging" underwater "pretty spurious".
He said: "You've got Titanic banging away with all its deteriorating structures.
"I have to say, from my own experience, using sonobuoys, you know pretty instantly what the sounds are - whether they're the sort of thing you're looking for.
"So I just wonder if they're actually slightly embarrassed by making such a story of this."
He went on: "To my mind, the Titan was going down to the Titanic.
"If that's where she was going, that's where you start your search and you expand it out in circles.
"And I'm really worried in case these spurious sounds may be taking people away from where the Titan might be."
It could be a "wild goose chase", he added.
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The Titan just before it launched. Pic: Action Aviation
The Titan went missing on Sunday as it descended towards the wreckage of the Titanic.
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.
Overnight, there were reports of "banging", but two senior Department of Homeland Security officials told NBC News, Sky's partner network, that it is more accurate to talk of "sounds" and "noises".