The commander of the Indonesian armed forces, Gen Moeldoko, told the BBC divers had been sent to investigate.
The pings were heard near where the plane's tail was found. Officials say the black box could have been separated from the rear part of the plane.
QZ8501 disappeared from radar on 28 December with 162 people on board.
No survivors have been found from the Airbus A320-200, which was flying from Surabaya in Indonesia to Singapore.
"We received an update from the field that the pinger locator already detected pings," said Santoso Sayogo, an investigator at the National Transportation Safety Committee.
"We have our fingers crossed it is the black box. Divers need to confirm. Unfortunately it seems it's off from the tail. But the divers need to confirm the position."
The "black box" flight data recorders are usually housed inside the rear part of the plane. They are designed to survive a crash and being submerged in water, and contain underwater locator beacons which emit the so-called "pings" for at least 30 days.
Finding them has been one of the top priorities for search teams as they provide crucial clues from the last moments of the flight before it came down.
The cause of the crash is unknown but the plane encountered bad weather and asked for a flight path change before communication was lost.
'Broken condition'
The BBC's Alice Budisatrijo, who is on board an Indonesian warship with Gen Moeldoko, says a crane has been deployed to pull the tail out of the water. An attempt yesterday was hampered by strong currents and low visibility.
The rear part of the plane was spotted on Wednesday by an unmanned underwater vehicle at a depth of about 30 metres .
Authorities said it was upside down and partially buried about 30 km (20 miles) from the point of last contact with the plane, off the coast of Borneo.
Gen Moeldoko said on Friday the tail appeared to be in broken condition.
Authorities have been pulling bodies and wreckage from the sea but progress has been slow due to high waves and stormy weather.
Forty-six bodies have been retrieved so far. Search teams believe most of the remains may still be inside the fuselage of the plane which has yet to be found.
