300E fresh air flapping noise

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 JB