My 1995 E320 89,000 mi started this 2 weeks ago. I used a stethoscope to trace it down to the air pump (located above the alternator on the pass. side of the engine). The links I've read tend to indicate that it is the bearing associated with the pulley on the electromagnetic clutch which is independently replaceable. I found a very reasonably priced site (ordered a full pump today and am waiting for the part) which will sell you the clutch/pulley or the whole pump.
www.mbpumps.com
If anyone is interested I wil post my experence with the site.