Our 1993 Merecedes 190 E (4 cylinder) is making a loud squealing noise when first cranked up. It started about 2 weeks ago and was minimal but now has grown to lasting about the first 5 minutes of the drive (my guess is that the car heats up enough and then the squealing stops).

I have been told it is the serpentine belt which is inexpensive to replace. I have also been told it might be the belt tensioner that may cost as much as $500. What do you think is the problem?