Sorry I can't offer any advice, but I've noticed the same problem with my '97 C230 (134 000 Km). However, I would describe the sound as a 'creak'. Mine seems to come and go, is dependent on the outside temperature (the cooler, the more noticeable), and primarily affects the rear suspension. Similarly, I have not noticed any decreases in performance.
I've been meaning to post this problem earlier, but due to the hotter weather up here in the Great White North, I haven't heard it lately and it's slipped my mind. Although that could be related to drinking too much beer at the cottage which may have negatively affected my memory (just reinforcing Canadian preconceptions).