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?