The Tensioner keeps tension on the belt, if it breaks it looses tension and you belt becomes loose. It's a commen problem on a 300E, needs to be replaced at some point. But go for this, get your belt replaced (if your not doing it) and have them look at the tensioner. If it's broken, they'll tell u and be ready to fork over some cash. I delt with it for like 2months it is prolly the most annoying sound ever.
Hope this helps