More underwater "banging" noises have been heard from the tourist submarine taking passengers to see the wreckage of the Titanic.
The US Navy cannot yet confirm if the noises are coming from the stranded Titan tourist vessel.
The Titan, which is operated by OceanGate Expeditions, hit the water at 8am on Sunday.
The Polar Prince, an expedition ship, lost contact with the vessel at 9.45am.
US Coast Guard Captain Jamie Frederick held a press conference to provide an update on the missing Titanic tourist submarineGB News
The five-person submarine was not reported missing to the Coast Guard until 5.40pm.
Captain Jamie Frederick, from the US Coast Guard First District, offered a glimmer of hope by saying sounds were heard underwater.
He added the sounds and ongoing search provided "hope".
Frederick said: "Several flights heard noises yesterday... the P-3 heard noises today as well.
An underwater camera has been used as part of rescue effortsReuters
"We don't know what the noises are, to be frank with you... they have been described as banging noises but they are inconclusive.
"The good news is we are searching in the area where the noises were detected."
His comments came after a Canadian P3 search aircraft heard similar sounds yesterday.
Navy experts have been analysing the sounds heard.
Captain Frederick also stressed the missing submarine is still being treated as a "search and rescue" operation rather than a "recovery".
He said: "This is a search and rescue mission 100 per cent, we are smack dab in the middle of search and rescue and will continue to put every available asset that we have in an effort to find the Titan and the crew members.”
Captain Frederick previously reiterated that the estimate of 40 hours is based on the initial report of 96 hours.
However, such an estimate would mean passengers have until around 10am (BST) on Thursday.
The Titanic tourist sub vanished in the Atlantic Ocean on Sunday, sparking an urgent searchReuters
The announcement means passengers have until around 10am (BST) on Thursday.
Cautioning the figure, Frederick warned: "That’s just one piece of data right."
British billionaire Hamish Harding, 58, is among those missing as rescue crews continue to search the Atlantic Ocean.
Shehzada Dawood and his son Suleman from Pakistan were also onboard the five-person expedition vessel.