e Chinese ship that detected a sound in the southern Indian Ocean consistent with a black box “ping” may have simply been listening to itself, experts say.

That possibility illustrates just how complicated the task is to locate the transmitters from the lost Malaysia Airlines flight. Australian Air Chief Marshal Angus Houston warned Tuesday — 32 days since Flight 370 vanished after takeoff from Kuala Lumpur — that other ships are being kept away from the search area to prevent any further confusion about signal noise.

"We can't have too many ships in the area, because when you are dealing with these transmissions, you need utter silence," Houston said. "It becomes a very noisy environment if you suddenly have several ships around there or ships dropping things in the water."

The Chinese announced Saturday they had recorded fleeting pings off Australia’s west coast, but their discovery occurred more than 350 miles from where the Australians say sustained and repeated pings were found Sunday.

Video of the Chinese ship Haixun 01 appeared to show an extra pinger on board. That pinger would just need to get wet for it to start transmitting — noise that could then get picked up by the ship’s search crews, said Anish Patel, president of pinger maker Dukane Seacom. The company supplies black-box beacons for Malaysia Airlines.

“It takes the slightest bit of moisture for that water-activated switch to automatically fire up,” Patel said.

He added that it’s “not best practice” during a search to keep another pinging device where you’re listening.

Video on China’s CCTV shows the crew of the Haixun 01 boarding a small dinghy and using a handheld hydrophone, which was lowered into the water on a pole, to listen for the pings from the missing jet’s two black boxes.

"We can't have too many ships in the area, because when you are dealing with these transmissions, you need utter silence."

Experts have said that while it’s possible that such a device could pick up pings from the ocean, it’s highly unlikely in this case. The maker of the hydrophone technology used by the Chinese said it's intended for shallower waters and requires the user to be much closer to the transmitter. The depth of the ocean in the search area is as much as 3 miles.

The Chinese said they recorded the fleeting pings on Friday and Saturday off Australia’s west coast. The signal’s frequency was recorded at 37.5kHz per second — the same that would be emitted by flight recorders and a noise that does not occur naturally in the ocean.

Australia also is dragging a ping locator in the search area, and officials said signals it picked up late Saturday and early Sunday were stronger and lasted longer than the ones detected by the Chinese.

Those noises haven’t been heard since. The signals would become fainter over time as the beacons’ battery life slowly dies out. That typically takes about a month.