Cam Belt?

I think I have a problem with my cam belt. I have a Fiat Seicento Sporting and its a 2002 model. I dont have a clue about its service history! Each morning I turn the engine on and when its cold, there is a whistling noise coming from the engine which is constant through mid range revving until the engine is warm from driving it after a few minutes then it is quiet! What do you reckon it can be?