The exhaust flex is one of the repairs on my om642. It bolts directly to the back of the turbo, connecting the exhaust port of the turbo to the next bits of the exhaust system (a catalyst, I think). On my w211 it was a straightforward DIY repair, but access to that part of the engine bay is wide open. About $200 in parts cost from an Atlanta MB dealer that does some mail order business. The part had to come from Germany, so it took a week.

On my car the symptoms were a dieselly smell in the cabin at idle and some soot on the firewall near the flex. Never noticed any change in the engine sound even though it is an exhaust leak.