The idler pulley is probably the problem. Not too hard to replace yourselft if you have the right tools. It is just visible in the picture above. These wear out and have to be replaced. I got one cheaper at the MB dealer. I just told heim I had a 2003 SLK320. to replace you turn the tensioner to remome the belt and then unscrew the center bolt on the pulley. Do the opposite to put back together.