A squealing alternator pulley is not only annoying, it indicates a problem with the car's belts or alternator. Left unattended, the problem that causes a pulley squeal will likely become worse and lead to expensive alternator or engine repairs.
A loose drive belt may cause the belt to slip at the alternator pulley, creating a squeal. Most modern cars use a serpentine belt system that is tensioned by an automatic belt tensioner, which cannot be adjusted manually. In these cars, problems with the belt tension are often caused by a worn belt that has stretched.
A worn belt can create a squeal even if it is properly tensioned. A drive belt should be soft and pliable, allowing it to bend as needed and providing a solid grip on the pulley. Worn belts will often develop hard spots, cracks, or have a glazed appearance, which requires belt replacement.
An alternator's bearings can become worn and create noises, including a squeal. An alternator with worn bearings will likely produce an insufficient charge to operate the engine and charge the battery. To test for worn bearings, remove the belt and turn the pulley by hand. If noises are heard or the pulley does not turn smoothly, the bearings are worn and the alternator should be replaced.