CNN — The actual sound is mundane, like the ticking of a second hand on a loud wall clock.
But it's the sound that searchers from around the world are desperately hoping to hear -- the noise of the pinger from missing Malaysia Airlines Flight 370.
The problem is, time's running out. The pinger's batteries will likely die after 30 to 45 days, and Friday marks the 28th day of the search.
So what happens if the pulse of the plane sputters out? Is there any hope left of finding the jet carrying 239 people? Here's what to know about the future of the search:
Let's catch up -- what exactly are pingers, and how do you find them?
Every commercial airplane is required to have pingers -- technically called underwater locator beacons -- to help find lost planes. One is attached to the flight data recorder; another to the cockpit voice recorder.
The pings sound about once every second. The noise is inaudible to the human ear, but devices such as towed pinger locators (TPLs) can hear the sound from 2 nautical miles away.
The pinger locator has a sensor that looks like a 35-inch, 70-pound yellow stingray. It can recognize the flight recorder's chirps up to 20,000 feet below the surface of the water.
The Australian ship is dragging a TPL on loan from the United States to help hunt for the plane in the Indian Ocean.
What are the challenges of hunting for the plane by pings?
Not only will the batteries powering the pinger die after about 30 to 45 days, the sound can be drowned out by weather, noise or silt.
Also, the pinger locator has to be towed slowly. It could take days to cover the 240-kilometer (150-mile) track identified by officials as the latest best guess for where the plane could be.
"It is a very slow proceeding search, 2 to 3 knots depending on the depth that you want the hydrophone, that tow pinger locator trailed at," said Capt. Mark M. Matthews, the U.S. Navy's head of TPL operations. "It's going to take time. ... Again, we're searching on what information we do have, our best guess at where it would have been lost. It's the best we can do at this time."
So is all hope lost in finding a plane after the pinger dies?
No. Take, for example, Air France Flight 447, which disappeared in 2009. A towed pinger locator searched for but couldn't find the plane, which crashed hundreds of miles off the coast of Brazil.
But two years later, searchers found the flight data recorder and the bulk of the wreckage using an autonomous underwater vehicle, or AUV.
What other kinds of high-tech gadgets can searchers use to find Malaysia Airlines Flight 370?
One of the Australian search ships has an underwater robot called the Bluefin-21, which can scour the ocean bed looking for signs of wreckage.
But the robot, on loan from the United States, hasn't been deployed yet. Matthews said the Bluefin-21 would only be deployed if the searchers get a clear fix on the beacons sending out the pings.
Searchers could also use autonomous underwater vehicles, which are typically used in the oil and gas industry to conduct deepwater oilfield surveys.
"The smaller ones are only going to go down to about 5,000 feet," analyst David Soucie said. "The next class is a much more expensive, much larger device. It's 15 by 25 feet because it adds a lot of battery capability and a lot of hydraulic capability."
One of the most sophisticated AUVs owned by Phoenix International was activated and flown to Perth, Australia, to help with the search for Flight 370. The device is yellow, 17.2 feet long and has an in-air weight of 1,600 pounds.
It can be lowered 20,000 feet below the water surface and travels 2 to 4.5 knots, using side-scan sonar to create a map of the seafloor. The rapidly moving probe is also equipped with a still camera.
"A picture will scroll, and you will see the seafloor be painted in front of you," said Jami Cheramie of C&C Technology, whose AUV has been called in to search for plane debris in the past.
Have these underwater vehicles found plane wrecks in the past?
Yes. AUVs played an instrumental role in finding the downed Air France flight, the plane wreckage of Italian fashion designer Vittorio Missoni off the coast of Venezuela, and the HMS Ark Royal, a ship sunk by a German U-81 submarine in World War II. The AUV provided black and white images of the wreckage site.
Will the mystery of Flight 370 be solved once the data recorders are found?
Not necessarily. The voice recorders have only two hours of recording capacity. And since officials believe Flight 370 flew almost seven hours beyond the point where something went terribly wrong, some crucial cockpit sounds have almost certainly been erased.
On the positive side, the depletion of the battery will not wipe out data. Data has been known to survive years in harsh sea water conditions on modern recorders.
CNN's Rose Arce, Rosa Flores, Mike M. Ahlers, Jethro Mullen and Paula Hancocks contributed to this report.