Chevy Silverado 1500 Strange Sounds From AC Unit

“I have a 94 Chevy 1500 pickup. not too long ago, i started hearing a “thump thump thump” noise when I start the vehicle, and shut it off. it lasts for about 10 to 15 seconds, it comes from under the passenger side of the dashboard. i do know it has to do with the heating/cooling. i just recharged my a/c system yesterday, and now it happens as well, when i hit the recirc button for my a/c i guess that’s the same as a Max button.”

OK, since it happens when you hit the recirc button, then it’s got to be the flap that goes from the open (outside air) position to the closed (inside air) position. The flap is opened and closed by an electronically activated vacuum motor that has a rod connected to the door flap. Check to make sure you don’t have any vacuum leaks in the system, as weak vacuum pressure can make it flap open and closed–BUT when the vacuum pressure builds up as the engine runs and sucks more, then the thumping will stop. I’ve seen that happen MANY times. It’s a real BEAR taking that dash apart, so if it works fine after a few seconds, I’d personally just live with it. BUT, if it’s driving you nuts, hopefully you have really tiny hands and arms to fit in there and check things out (otherwise you’ve got to take the whole dash apart on that side to see what’s going on.)