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.