93 suburban heaterdoor noise

I have a 93 suburban that when you turn the ignition on, the recirculating heater door thumps for several seconds also does it again when turning the ignition back off it does same any advice would be appreciated. this is a 4 wd with 454/7.4 engine.

I have a 93 suburban that when you turn the ignition on, the recirculating heater door thumps for several seconds also does it again when turning the ignition back off it does same any advice would be appreciated. this is a 4 wd with 454/7.4 engine.
I had the same problem. The electric motor powers the doors via 2 lever arms that are spring loaded and frequently pop off the door arms when turning the key on and off. This arms are replaced with a modified design from OEM Chev parts that eliminate this problem. You will know this when you buy the replacement parts from Chevrolet. Beware you don't break the split plastic arm of the main lower heater door or you will have an additional 2 hours of labor to replace that door-if you remove the lower lever arm carefully this won't happen to you.