I've never owned a manual Mercedes but the noise you describe is familiar on other vehicles. It is unlikely that the release bearing would make a noise in the circumstances you describe, since this bearing is only spinning when you have the clutch depressed.
As Duke2.6 says, the bearing on the gearbox input shaft may be worn. I have run for tens of thousands of miles in vehicles with this problem, and never had a failure. I can't guarantee this will be true in your case of course, but so long as the gearbox is quite in normal running, I wouldn't worry.