Heater problem
1993 Other Chevrolet Models 6 cyl Two Wheel Drive Automatic

I have a chevy wt truck 1993 and the black vent systen under the dash everytime I start the truck or shut it off there is a clicking noise coming from that black box area under the dash, I have stuck my hand up in there and seems there is door that is trying to move but cannot so what do I do to fix this???