As crews scour the Atlantic Ocean for the missing submersible, the U.S. Coast Guard said it's searching in the area where underwater noise was detected.

The unidentified noise was detected Tuesday night and Wednesday morning.

The data on the noise was sent to the U.S. Navy to be examined, according to the Coast Guard.

"We have to remain optimistic and hopeful when we're in a search and rescue mission," Capt. Jamie Frederick, the First Coast Guard District response coordinator, said at a press conference on Wednesday afternoon.

The deep-sea vessel submerged on Sunday morning with five people aboard and a 96-hour oxygen supply, according to the Coast Guard. That amount of breathable air is forecast to run out on Thursday morning.

When asked about the search becoming a recovery mission, Frederick told reporters: "We're not there yet."

One of the ships the Canadian Coast Guard sent to the rescue effort, the John Cabot, is equipped with advanced, deep sonar, said Joyce Murray, minister of Fisheries, Oceans and the Canadian Coast Guard.

"We've sent so many assets to the search team to help," Murray said. "Authorities still have hope in the mission."

-ABC News' Will Gretsky

Mike Reiss, who has done four, 10-hour dives with OceanGate, including one to the Titanic, told ABC News his sub lost contact with the host ship on every dive.

"Every time they lost communication -- that seems to be just something baked into the system," he said.

With no GPS, Reiss said it took his crew three hours to find the Titanic despite landing just 500 yards from the ship.

Reiss said he signed "a waiver that mentions death three times on the first page.”

"It is always in the back of your head that this is dangerous, and any small problem will turn into a major catastrophe," he said.

He said the submersible is built simply and is "just propelled by two fans on the outside."

"Even I was able to steer and navigate the sub for a while," he noted.

Reiss said his greatest fear was that the sub wouldn’t be able to release the weights that force it to submerge once it was time to rise to the surface.

-ABC News' Gio Benitez and Sam Sweeney

Digital marketing tycoon Chris Brown originally planned to go on Sunday's submersible tour of the Titanic wreckage but withdrew due to safety concerns. His friend, Hamish Harding, is among those aboard the missing vessel.

Brown, who described himself as a "modern day adventurer," admitted that he knew "very little" about the tour operator, OceanGate Expeditions, prior to signing up. But he said the deep-sea trip initially sounded "like a great idea."

"The Titanic's obviously an iconic wreck," Brown told ABC News' Michael Strahan during an interview Wednesday on "Good Morning America."

"OceanGate had put forward this program to go down and do a 3D scan of the wreck," he added. "So it's a chance for an expedition, exploration and adding a bit of science into the situation."

Brown said safety concerns ultimately led him to pull out of the trip, but he declined to go into detail.

"I'm not really sure that this is the time to be going into that sort of thing," he added. "I think that the focus right now has to be on trying to rescue these people. It's not fair on the families and friends to be making speculation about what might've happened or how it happened at this point."

Brown said he feels "no" apprehension about joining similar projects in the future but noted that it's important to "think about all the risks."

"If you don’t have an appetite for those risks, then you might not go ahead," he said. "You may try and mitigate those by bringing in some expertise from outside. You might have other risks or dangers that could be mitigated by changing the time of when you go out there."

A former employee of OceanGate alleged in a 2018 counterclaim lawsuit that he was fired for raising concerns about quality control and testing of potential flaws in the same experimental submersible that went missing this week.

David Lochridge, an engineer and submarine pilot, claimed in his counterclaim against OceanGate that he was hired in 2015 by the Everett, Washington, company to ensure the safety of all crew and clients during the submersible and surface operations of the vessel called Titan. But when he expressed concerns about the design and testing of the minisub's hull, he said he was terminated by the company.

OceanGate had initially sued Lochridge alleging, among other things, breach of contract, fraud and misappropriation of trade secrets -- all claims he denied.

In its lawsuit, OceanGate accused Lochridge of breaching his contract by discussing the company's confidential information with the Occupational Health and Safety Administration "when he filed a false report claiming that he was discharged in retaliation for being a whistleblower."

Read more here.

-ABC News' Bill Hutchinson