Crews searching for a sub that went missing while taking five people to the wreckage of the Titanic continued to hear noises Wednesday and were "actively searching" the area, the U.S. Coast Guard said.

Overnight, the agency said a Canadian search plane detected noises underwater in the search area Tuesday and crews were focused on finding the origin of the sounds. Coast Guard Capt. Jamie Frederick said a plane heard the noises Wednesday morning as well.

"With respect to the noises, specifically, we don't know what they are, to be frank with you," Frederick said at a briefing Wednesday. "...We're searching in the area where the noises were detected."

He said the team has two ROVs — remotely operated underwater vehicles — "actively searching," plus several more are on the way and expected to join the search operation Thursday.

Search flights were scheduled to continue throughout the day and into the evening, Frederick said.

Carl Hartsfield of the Woods Hole Oceanographic Institution said the noises have been described as banging noises, but he also said it was difficult to discern the source of noises underwater.

"They have to put the whole picture together in context and they have to eliminate potential man-made sources other than the Titan," Hartsfield said, referring to the sub's name. "...The team is searching in the right area, so if you continue to do the analysis, look for different patterns and search in the right area, you're doing, you know, the best you possibly can do with the best people on the case."

The sub's disappearance on Sunday has spurred a massive response from the U.S. and Canada as search crews rush to find the missing group in the north Atlantic Ocean. Five vessels were searching for the sub on the water's surface as of Wednesday afternoon, and that number was expected to double to 10 within 24 to 48 hours, Frederick said.

A Canadian research vessel lost contact with the 21-foot sub an hour and 45 minutes into its dive Sunday morning about 900 nautical miles off the coast of Cape Cod, Massachusetts. It had been expected to resurface Sunday afternoon.

The size of the search area has expanded to approximately twice the size of Connecticut, with an underwater depth of up to 2 and a half miles, Frederick said.

Frederick continued to express optimism about the search in its third full day.

"When you're in the middle of a search and rescue case, you always have hope," he said. "That's why we're doing what we do."

Frederick said on Tuesday that the sub could have around 40 hours of breathable air remaining, but declined to provide a new estimate in Wednesday's briefing, saying that the remaining oxygen was "a dialogue that's happening" but not the only detail being considered.

"This is a search and rescue mission, 100%," he said. "We are smack-dab in the middle of search and rescue, and we'll continue to put every available asset that we have in an effort to find the Titan and the crew members."

Frederick acknowledged that sometimes search and rescue missions aren't successful and officials have to make "a tough decision" about continuing efforts.

"We're not there yet," he said. "But, if we continue to search, potentially we could be at that point, but, again, we're not there yet."

In:
• RMS Titanic
• United States Coast Guard
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Alex Sundby

Alex Sundby is a senior editor for CBSNews.com

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