Dear Car Talk:

I have a 2002 Jeep Wrangler Sport. It's in great condition, but it's making a noise I cannot live with. After a few miles on the road, it starts to make a "teapot" whistling noise. When I press on the gas, it goes away, but as I release the gas, it starts again. Any idea what it is? -- Becki



Unfortunately, yes. Well, if you're really lucky, it could be something as simple as a vacuum leak. But in that case, Becki, you should be able to duplicate the noise by pulling over and revving the engine with the car in park. If you hear the noise only when the car is in motion, then I'm afraid it sounds ominously like differential whine.

My brother had an old Chevy Suburban, and for months he kept pulling over on the highway, thinking there was a cop behind him blaring his siren. Turned out it was the differential.

The differential is a simple box of six gears that -- through some sort of magic we mortals don't understand -- allows the wheels to turn at different speeds.

Why do they need to turn at different speeds? So the car can turn. If you take a Matchbox car, paint the wheels black and then drive it in a very tight circle on your kitchen table, you'll see from the tracks that the outside wheels travel farther than the inside wheels. And since all the wheels arrive at the same time, the outside wheels must be turning faster. The differential is what allows the rear wheels, in your case, to get power while turning at different speeds.

By the way, sorry about your kitchen table.

Anyway, Becki, what you're describing are the classic symptoms of a failing differential: a howling/whining noise that changes depending on whether you're accelerating or coasting.

Unfortunately, you have two differentials on this car (because it's four-wheel-drive). The most likely scenario is that the damage has already been done, and one of the differentials has gears that are worn out.

But before you jump to that conclusion, there's one thing you can try: Have a mechanic check the differential fluid. If by some chance one of the differentials is low, and it hasn't been low for too long, you can try filling it back up.

If the noise goes away after that, trade in the car immediately, while it's quiet. Good luck.