I'm not real familiar with the newer MB models, but if the compressor has a clutch it may be the bearing in the clutch. If that's the case you can remove the clutch and put a new bearing in or you can replace the clutch itself which would come with a bearing already pressed into the clutch assembly. On the 124 and 163 chassis MBs I've done this job without removing the compressor.

Find out if it's the clutch or the compressor.