take a look at this:
http://got.net/~danj/fiat/pulley.jpg (this part is off a saab, not a fiat)

sometimes the pulley that drives the belt fails and makes the same sound you describe.

what kind of car is it? (year, model, engine)
if the car uses the 'serpentine' style belt (kinda flat) you may also want to check the idler wheels/bearings that tension or re-direct it.

don't ignore this problem. something as simple as a bad belt will bring me a head gasket job at least once or twice a month.