Matt is probably right. If you can turn the alternator pulley with the engine off the belt is too loose. Alternator belts love to squeel. If not sure just run a bar of soap along the v portion of the belt with the engine stopped of course. It acts to temporarily increase traction of the belt and again if the squeel is eliminated by it adjust or change the belt if worn or damaged. I find in most cars It is a common problem and perhaps one of the reasons why they went to self adjusting serpentine belts years ago. I also have always wondered if it is a squeel or that a shreik might be a better description but everyone always seems to call it a squeel.