My 1988 300E has a noise in the dashboard associated with the fresh air
system. If it's cold outside and I am in the economy(non air
conditioning) mode, with the thumb wheel thermostat in the heating
position, the unit works perfectly fine. that is no noise. In warm
weather and wanting just some air flow wanted with fan on, the system
makes a pulsing flap, flap, flap noise every half-second. Very annoying.
To get it to stop, I must turn the system off, ie no fan spinning. What
causes this and what's involved with fixing it? Thanks for your help