A squeaking fan belt is an indication of an underlying problem that you need to diagnose and fix. The fan belt, which is the s-belt (serpentine belt) in a newer car, or one of the v-belts in an older car model, will squeak, squeal or make other noises if it is worn or loose, or has had engine fluids like oil or coolant spilt on it. In some cases on older cars, a squeaking v-belt can be temporarily silenced by tightening it, but in most cases a squeaking fan belt will have to be replaced. Never lubricate a belt to stop noise.
Put on safety goggles. In older car models with v-belts, use an adjustable wrench to loosen the pulley that is applying tension to the squealing belt. In cars with a serpentine belt (models built after the late 1980s), use an adjustable wrench or belt tensioner release tool to release tension on the belt tensioner. Be aware that the tension can be released violently, which can cause the wrench to jam, crush your fingers, or damage nearby engine parts. With the tension released, remove the s-belt.
Inspect the pulleys and nearby parts for damage. Pulley, tensioner and fan bearings should not wobble. Check the fan blades for damage, and shake the fan to be sure the water pump bearings are not worn. Look for signs of leaking coolant, oil, brake fluid or transmission fluid that could be dripping onto the belt. Replace any leaking seals, hoses or worn out parts before putting on the new belt.
Clean the pulley grooves with a wire brush and brake cleaner. This will slightly roughen the surfaces, which may have been polished smooth by a slipping belt. If the belt does not have sufficient friction, it will not be able to transmit power from the engine to the fan or other accessories, and engine performance will suffer.
Install the new belt, taking care to seat it well in each of the pulley grooves. Tighten the belt tensioner with the wrench or tension tool. Start the engine and listen for noises. Check for leaks in any hoses or other fluid-carrying parts that you replaced.
Replacing a worn belt can be dangerous if you are unfamiliar with what you are doing. It is much cheaper to pay a mechanic to do it for you, if you have any doubts, than to pay for medical bills if you hurt yourself. If your old belt is still intact, keep it in your boot for an emergency replacement. You should never rely on a worn belt for continuous use, but having it for a temporary spare could prove convenient.
Always wear safety goggles when you are using tools. The belt tensioner can be very tightly set, so take care when releasing tension to avoid a sudden release that could injure you or slam your wrench into nearby engine parts. Never try to stop a belt from squeaking by lubricating it. Lubricating a belt may stop the noise, but it will make the belt slip and keep it from transferring power. The result could be a very quiet ride until the battery is drained or the engine overheats because the s-belt is slipping.