Or you could try lubricating it firstItem #1 is related to the serpentine belt, it is slipping slightly. Just replace it wit a new one and check the tensioner pulley too.
Mine was squeaking about 2 months ago and i sprayed it with some silicone and haven't had a problem since. It sure beats just replacing the belt for the hell of it. The recent cold weather this winter has caused a lot of belt squeaks for bimmer owners. Some just need a lube, others need a new belt/tensioner/adjustment. depends.