There are several possible problems indicated by a loud squealing noise when a car is started. The most common problem is a worn or loose belt.
The serpentine belt, also sometimes called the accessory belt, drives the alternator and various pumps, and if it is loose or worn, it begins making a squealing noise. While light wear is not a cause for immediate concern, the belt should be replaced immediately when the wear is noticeable. If the serpentine breaks while the car is moving, it may damage the parts it is attached to. If the belt is not worn, the owner should check the tensioner to ensure the belt is tightened properly.