The belt will usually squeal loudly if its loose or has fluid on it making it slip or is worn out and this is most evident when it is cold started. Happens very commonly around the neighbourhood especially in the mornings when cars are going out to work or school duties. I find this in W124s mainly, maybe they are the most popular in my neighbourhood. Loose belt, tensioner problems, worn out belt , whatever, get it checked by your trusted techie or you will be in for a lot more trouble as they will find "invisible and hocus pocus" breakdowns in the rest of the car which may really empty your pockets overnight!