This can be caused by dust sheilds (tisk tisk if you have them on the car)/. If not I would think that it could be the parking brake shoes. Try pushing the pedal in one click and see if the sound changes.

------------------
Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM