Outside air vent 'thumping'

Hear a small 'thumping' in dash (like tapping your hand on the dash) when push button to open outside air vent. Thumps 9 times, then makes other sound, then starts up again...9 times, then other sound. Repeats.

Stops when push button to close outside air venting. No other 'symptoms'. When run fan with outside air vent open (while thumping is going on), can hear air pulsating with each thump.

What would problem be? What would I be up against to get it fixed?

Thanks!