banging noise under dash

[michigan2500hd]

1994 chevy 1/2 ton, 4.3l. auto, work truck, 2wd

Lately every time i start my truck there is a loud banging noise that lasts about 15- 30 seconds. Have so far traced it to the back of the blower motor or at least that general area. This happens when i turn the truck off as well. Kind of sounds like the damper in the duct work just slapping open and closed many times. This sound can be heard by people walking anywhere near the truck,lol. I had hoped the problem would go away untill it warms up from the single digits but it has stayed. Is there a control that would cycle a flapper in the duct work that may have gone bad? should i remove the blower motor and see if i can see anything in there? thanks

[bobcat5544]

Most likely it is the blend door or air recirculation door actuator gone bad. Normal for this truck. it will make the ecact same sound that you are talking about. try to change the setting on the ac/heater control heat one of the settings will make it stop and tell you which door it is.