Soldato
Seems likely what's happening.A submarine search and rescue expert in Australia, Frank Owen, tells the BBC his “confidence went up by an order of magnitude” when he heard reports of banging being detected by floating sound detectors.
“There's a couple of reasons for that,” he explains. “Firstly, on board this craft is a retired French navy diver. He would know the protocol for trying to alert searching forces… on the hour and the half hour you bang like hell for three minutes.”