'94 1/2 ton-thumping under dash

I've got a thumping sound coming from under the passenger side dash every time I start and shut down the engine . The thumping last about 20 seconds both at starting and shutting off the motor. I thought it had something to do with the vacuum controls of the a/c system but I removed the vacuum line where it goes though the firewall and the thumping is still there. What is under the dash that could be making this noise?

I've got a thumping sound coming from under the passenger side dash every time I start and shut down the engine . The thumping last about 20 seconds both at starting and shutting off the motor. I thought it had something to do with the vacuum controls of the a/c system but I removed the vacuum line where it goes though the firewall and the thumping is still there. What is under the dash that could be making this noise?

Could be the temp door actuator. They are electric not vacuum operated and they can get worn out. On my 90 K1500, I had to replace it a couple of years ago.

//2000CAYukon