1994 GMC truck thumping noise under dash

My 1994 GMC 1/2 ton truck started making a loud thumping noise under the right side of the dash. It happens everytime I turn on or off the key. The thumping will last about 20 seconds and then stop. It sounds like the heating fan is hitting something in the heating cowl area. Could there be a relay or vacuum device that is supposed to open or close a door that isn?t working properly? any ideas? thanks