I tracked down what I would describe as a "rythmic knocking" tied to the ventilation system this last weekend to the duo-valve on the passenger side of the engine compartment [2001 CLK 320]. It sounded like something trying to cycle unsuccesfully every second or so, very noticable "thunk ... thunk ... thunk" that sounded like it was coming from the center of the dash from the driver's seat. I live in AZ where this would be closed most of the year, but starting about 2 weeks ago the morning temps were low enough the system tries to provide a little heat and it started the "loud enough to be annoying" knock. A search led to a few threads, all say the valve is "not repairable" and would need to be replaced. Part number is 001 830 34 84 and runs ~$230. Since mine is functioning but noisy, I thought maybe I could pull it out and see if one of the valves is stuck shut or not cycling properly and can be cleaned up. Anyone have advice or hints on this particular unit?

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