Mine did that and then gradually stopped. Quiet now. Not sure why. There are shims for it (washer rally, I think). And you can get a rebuilt from a dealer at a third of the cost of a new one (Tom Hanson at Caliber Motors). There is a special puller MB# 126 589 06 33 00 to get it off and you will need a pin wrench MB# 000 589 00 05 00 (though I think this is a relatively standard tool). The procedure with the compressor off the car says you can knock the pulley off with a plastic hammer, so you coudl probably do that even if it is on the car (but might not be very much fun). I would not let it languish if it is noisy. If it fails your serpentine belt fails and car becomes inop.
1986 300E Anthracite + ECodes + MB Mileage Award