On some non-MB cars I've owned, I've noticed over time that brake dust accumulation will cause a noise such as the one you describe. You might consider pulling the pads; then cleaning everything. Obviously there could be other issues.
What I'm suggesting is quick, simple; cheap.
As far as turning MB rotors goes, I think M.B.DOC had the final word on this issue. Check out what he said in the following post.
http://www.peachparts.com/shopforum/showthread.php?threadid=12589