The development came as the clock was ticking down on hopes of finding alive the 44 crew members now missing for a week despite a massive search of surface and seabed, amid fears their oxygen had run out.
The ARA San Juan would have had enough oxygen for its crew to survive underwater in the South Atlantic for seven days since its last contact, according to officials. At 0730 GMT Wednesday, that time had elapsed.
Navy spokesman Enrique Balbi told reporters a "hydro-acoustic anomaly" was detected in the ocean almost three hours after the last communication with the vessel on November 15, 30 miles (48 kilometers) north of its last known position.
Asked if the noise could have been an explosion, the spokesman declined to speculate, saying only: "It has to be corroborated and looked into."
Balbi added: "We are in a very dangerous situation, and one that is getting worse."
Information about the unusual noise became available Thursday after being relayed by the United States and "after all the information from all agencies reporting such hydroacoustic events was reviewed, Balbi explained.
"It would have been a very loud noise" and one that could have been an explosion, a former sub commander told AFP privately.
High seas and poor visibility in the South Atlantic have hampered the search since it began, around 200 miles (320 kilometers) off the Argentine coast. Waves have towered as high as six meters (20 feet).
The conditions have fed hopes that the vessel may be on the surface undetected.
Despite the mechanical problems it reported during its last contact last Wednesday, the crew could survive indefinitely if the sub retained the ability to rise to the surface to "snort" or replenish its air.
Conditions improved Tuesday, but the forecast for Thursday is once again poor.
The 34-year-old German-built diesel-electric submarine that was refitted between 2007 and 2014 had flagged a breakdown and said it was diverting to the navy base at Mar del Plata, where most of the crew members live.
It didn't issue a distress call, however.
Jessica Gopar posted a moving letter to her husband, San Juan crewman Fernando Santilli, father of their one-year-old baby, on Facebook.
"Hi, Fernando. I don't know if this finds you calm or desperate. Every day here becomes harder. There are moments of hope and great distress."
"I am surrounded by family, your colleagues, acquaintances and friends, there is not moment that we do not pray for your rescue. Today has to be that day," she wrote.
The sub's disappearance has gripped the nation, and President Mauricio Macri visited the relatives -- who have endured days of false hopes -- and prayed with them.
US President Donald Trump offered his support Wednesday, tweeting: "May God be with them and the people of Argentina!"
Underwater sounds detected by two Argentine search ships were determined to originate from a sea creature, not the vessel.
Satellite signals were also determined to be false alarms.
"A light begins to shine, and then it goes out," said Maria Morales, the mother of one of the missing sailors.
"There is a curtain of smoke, we don't know anything," said Elena Alfaro, whose brother is aboard the submarine.
"It doesn't make sense that so much time has passed without anyone knowing anything," she added.
"The hours go by. We're hoping for a miracle. I don't want to bury my brother, I want him with me. I feel he'll come back, but I am aware of time passing."
Argentina is leading an air-and-sea search with help from several countries including Brazil, Britain, Chile, Colombia, France, Germany, Peru, the United States and Uruguay.
The defense ministry said the search area could be expanded sevenfold, though it was already large.
Pain, helplessness, hope
The incident has recalled recent submarine disasters, perhaps most prominently that of the Kursk, a Russian nuclear sub that caught fire and exploded underwater in 2000, killing all 118 on board -- some instantly, others over several days.
An accident aboard a Chinese sub in 2003 killed 70 crew, apparently suffocated after what Beijing termed "mechanical problems."
Among the ARA San Juan's crew is Argentina's first female navy submariner: Eliana Krawczyk, 35.
Cards, banners with slogans and placards have been strung up on the outside of the Mar del Plata base's wire fence, expressing solidarity with the families tensely waiting for any news.
"There's a mix of feelings: pain, helplessness, at times hope," Morales said.
"The feeling is that they will come back, that we will tell ourselves today, 'They are back.'"//AFP