Thumping sound coming from around the blower

[marshallsmith27]

I have a 1993 Sierra and when I get in the truck and take off once I build up enough speed theres a noise under the dash near the blow that sounds like a door that slams. Not sure where exactly its coming from since I cant drive and have my head under the dash at the same time. when it slams you can feel fresh air from outside come onto my feet. I assume its the blend door but all the stuff I see online is of the blend door actuator clicking and stuff. Its starting to get hot here and I would like to fix this soon. My AC works just fine. A few days ago I accidentally blew some leaves into the squirrel cage so no I have a reason to get under there and get those out and fix this problem once and for all. so what do yall think this noise is. it does it once and no more until you get back into the truck again later

 

[highwaystar]

I think it's the recirculation actuator, mine was popping so I unplugged it, will replace later as it's hard to get to. There is a thread on here showing how to replace.

 

[marshallsmith27]

Mine only does it once you hit around 25-30 mph. it kind of reminds me of wind blowing your door closed at home.I am not sure whats down there to even start to fix it

 

[marshallsmith27]

I forgot to mention it does it once. it doesnt keep slamming.

 

[pressureangle]

Probably the gear in the actuator is broken, and has just enough traction to close the door; when the wind hits it, spreads the gear and opens the door until the actuator closes it again when you shut it off. Do a little testing, see if closing the windows up tight or having them rolled down makes a difference in the speed it happens.

 

[marshallsmith27]

Thats a great idea. I will definitely try the window thing. I didnt think about there being just enough wind to help it close.

 

[someotherguy]

When the linkages on the vent position actuator were gummed up/crudded up on my '94, it would thump into place after a while. Cleaning the crud off those and spraying a little fresh white lithium grease so they moved smoothly again solved the issue.

Richard