Ok, let me make sure that I understand you. You can hear it running with your ear to the vent? Is it constantly running with/without the CCU on? When I turn my car on I don't hear it. If I turn on the CCU all I can hear is air coming through the vent. I'll pull the glove box and take a closer look.

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1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k