What is your car telling you when it thumps, pings, or hisses? Should you head for your mechanic or just ignore it and hope for the best?

In its March issue, the car-whisperers at Consumer Reports take away some of the mystery by identifying which noises drivers should turn down the radio for.

When you hear a noise that's unusual -- a whine, a creak, or a squeal, for example -- consider it an alert that something has changed. By paying attention to those sounds, you can often catch a minor problem before it becomes a major one. Here's what to listen for:

• Squealing brakes. If you hear squealing only during braking, especially light braking, it's probably caused by a vibration at the brake pads. Squealing doesn't affect your braking performance and, while it's annoying, it is usually nothing to worry about and can happen even with new pads. If you hear a similar high-pitched squeal from the brakes while the car is moving but it stops when you step on the brake pedal, it can be a sign that your brake pads have worn down and need changing soon.

What to do. Have a mechanic inspect the brakes. In the case of the squeal during braking, he may be able to apply a lubricant to quiet the vibration.

• Squeals under the hood. If you hear this while revving the engine or when first starting it while cold, the noise is often due to a slipping drive belt. The belt could need adjusting or it could be glazed (the sides look shiny), which means it requires replacement.

What to do. Have a mechanic look over the drive belts and replace them if necessary.

• Light rattling in the engine. This might be pinging, which can sound like tiny marbles bouncing around inside the engine, and usually occurs while accelerating or climbing hills. It's often due to using gas with too low an octane rating and can begin occurring in older engines because of carbon buildup. Severe pinging can damage the engine.

What to do. Check your coolant temperature gauge. If the temperature is normal, try using premium gasoline. If that doesn't correct the pinging, have the car checked by a mechanic.

• Rhythmic thumping. If the noise increases and decreases with the speed of the car, it's probably a tire problem, such as torn rubber in the tread, a bubble in the sidewall, or a flat spot.

What to do. Have the tires inspected as soon as possible. Torn rubber or a bubble could lead to a blowout, so get new tires. A flat spot is annoying but not serious.

• Hissing beneath the hood. It's most likely a vacuum leak, caused by a cracked or disconnected vacuum hose. Your engine could also be running or idling rough and the "check engine" light could be on.

What to do. If it's a disconnected hose, you may be able to reconnect it yourself. Or take the car to a mechanic.

• Grinding sound from brakes. This means you've waited too long to replace your brake pads, your car is unsafe to drive, and your repair bill has just jumped to another level. The brake pads have worn completely through and are grinding against the brake rotor, which probably also needs to be replaced.

What to do. Stop driving and have the car towed to a mechanic.

Bear in mind that taking your car to a mechanic is no guarantee that your problems will be solved. ConsumerAffairs.com receives loads of complaints from consumers about the treatment they get at the hands of some of these "pros." Among them:

• Mark of Ewa Beach, HI, thinks he got ripped of by his local Pontiac dealer. "$250 to replace a car battery. (This is the third one on a four year old car); $425 to replace rear disk brakes only; and $328 to replace an ignition key. These are ridiculous service fees!"

• Ashleigh of Montgomery, AL, tells us of an unsettling experience with Ralph's Auto Repair. "Mr. M agreed to replace my engine in my 1999 Grand Jeep Cherokee for 1850.00. We made an agreement that if I gave him 1500.00 up front he would release my truck when it was repaired and set up a payment plan for the remaining balance." Ashleigh says he gave Mr. M 1500.00 as he requested, but now, "he has been avoiding my calls and has not repaired my truck or refunded me my money. I have been without a vehicle for four months now. I've had to pay friends/coworkers 60.00 a week to take me back and forth to work."

Amid growing concerns about the auto repair business, Congress is moving to guarantee consumers get a fair shake when they take their vehicles to be fixed.