1. Does your belt rattles or makes noise. The tensioner causes that problem most of the time, specially if it is the orginal.
2. LCA=Lower control arm. Is your suspension making any noises? How does it drive? If it is cracked, change it soon.
3. Door check straps are the mechanism that controls the "swing" of your doors when you open or close them. Do they make noise when you open or close. The part is about $12 a piece. Check the DIY article on top of this page. It has pictures.
If you are out of warranty, I would highly recommend to look for a honest MB independant mechanic. Don't go to the dealership unless if the last resource $$$$$$$$$$$$$$