One of the unsettling things about owning a Mercedes is that things that look simple often aren't. For instance the cabin air fan speed knob appears to function as you would expect except that sometimes some inner intelligence takes over and makes the fan speed up for no good reason.
I have read that the ventilation system (E320 CDI yr 2000)is a closed-loop system with a temperature sensor in the pod situated near the rear-view mirror. Presumably this is affected by whether the central vents are directed towards it or not.
Closed-loop systems are good until something happens to break the feedback. I can only assume that this is happening when I am driving the fully warmed up car in normal temperatures at normal road speeds and suddenly am met by an increased noise from the fan and fierce winds from all the vents in the car.