'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?