The serpentine belt, or fan belt, as some call it, transfers power from the engine crankshaft to the various engine accessories, such as the alternator, power steering pump etc. When the belt doesn't maintain constant adhesion to the various pulleys, it begins to slip and squeal. Often, the belt squeal is caused by moisture on the ribbed side of the belt.
Checking Things Out
Start the engine and raise the hood. Remember to keep your hands and clothes away from the engine and components.
Use the flashlight to carefully look for a place where you can spray the belt with WD-40 in such a way that the ribbed side of the belt is covered just before it travels under a pulley. Oftentimes, the exact place where the squeal occurs can be determined by listening to the sound and watching for a correlation between the sound and belt travel.
Squash the Squeal
Make sure to put on your safety glasses and be careful not to stand in line with the belt, as the spray will tend to shoot out in a wake and can stain clothing.
Spray just enough WD-40 onto the belt to lightly cover the area where the squeal occurs. Short, sharp blasts work best to ensure the belt is not saturated. Oversaturating the belt will result in severe slipping, which can permanently damage the belt. WD-40 is a water displacement lubricant and should remove the moisture from the belt ribs. In most cases, removing the moisture will stop the squeal.
Allow the engine to run for a few minutes, then spray rubber dressing, also known as belt dressing, onto the belt in the same manner employed above. This will treat the belt and remove any further contaminants from the ribbed surface. As with the WD-40, do not oversaturate the belt. Continue to run the engine for a few minutes to verify the squeal is gone.
If All Else Fails
If the squeal does not stop, or returns quickly, the serpentine belt is glazed. Glazing of the belt occurs after a prolonged period of slipping across the surface of one or more pulleys and can usually be seen as a reflective sheen on the ribbed side. A glazed belt will squeal, as it can no longer maintain a grip on the pulleys, and must be replaced.
Once you've determined you have to replace the belt, make sure to memorise the routing before removing it. Most vehicles have a routing diagram somewhere in the engine bay, usually near the radiator or on the underside of the hood. Before removing the belt, verify that every pulley is turning. Often, a glazed belt is a direct result of a seized pulley, and often a symptom of a larger problem.