carkarl

The part about using the windshield washers seems like an important clue. Isn't it the recirculate flap that would engage temporarily when running the washers? What about when you put the transmission in reverse? I think many of the newer Audis automatically engage recirculation while reversing, but I am not sure if that was already done in the B6 generation...

david9999

The fan speed does regulate itself based on the temperature difference between the inside cabin temperature, and the set temperature, and whether there is sunlight fallling on the the light sensor.
This would cause a change in fan speed.
If you use a vagcom, it should show a problem with for a solenoid or motor.
Maybe it is the motor that adjusts the flow over the heater core and the AC. I am just guessing.

Niar

Vagcom doesn't show anything.