If you don't get somewhere on this pretty quick, and you have the time and inclination to do so, I would recommend a visit to
Continental Imports in Gainesville. One of their principals is a member here with the username of stevebfl. His name is Steve Brotherton, and he really knows his stuff. The shop is a Bosch Authorized Service Center, so they are equipped to handle intricate Diesel related stuff. What I posted before is simply anecdotal and may or may not have a bearing on your engine's problem. It was simply to say that parts can be bad out of the box, and techs don't often think of that. And yes, you described my noise perfectly.
Welcome to Mercedesshop, and be sure to let us know how this thing turns out.