My ventilation system is fully functional, but tends to act irratic and make a thumping noise. When this happens, the fan will also be affected. It does it most when the selected temperature is close to the cabin temp. The fan will slow/sop, the thumping noise can be heard, then it usually resumes normal operation. Sometimes it doesn't and I have to cycle the system off & back on. I noticed a large "vacum pump" mounted just behind the controls. I think that it is what makes the noise. All the vent doors seem to operate noraml along with heat/cool functions. Any ideas?

Two things that I have seen cause this are a vacuum leak and a faulty push button assy.

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Benzmac:
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Thank you very much for the rapid response. I have feared the control unit to be at fault since I already did a once over on the vacum hoses. Usually when you have a list of possibilities, it is the most expensive one that ultimately solves the problem.