Phillip,

It sounds like your belt is loose as I think you suspect. Do a search on serpentine belt tensioners for the M103 and M104 for the info you need. Check to see that the tensioner is set properly. If the indicator points to the proper mark and the belt feels loose, then the tensioner has failed or will soon fail. There is no end of information on this site on how to replace the tensioner.

Sixto
91 300SE
81 300SD