Hello, new member, just bought a 1982 El Camino the Saturday before Father's Day. When I turned the vents on for the first time, there was a strange "thunk" sound. I was wondering if this is normal, and if there is anything I could do to fix the problem, or if there is nothing I can do?

That would be your a/c vent/defrost mode door sticking. Vacuum moves the door to the a/c position, and when you shut the car off, the door relaxes to the defrost position until you start the car again..creating vacuum and moving the door back to a/c. The doors tend to stick, (very common amoung g-bodies.) Short of removing the distribution box behind the dash and disassembling to access the door the only band-aid fix is to spray a penetrating lubricant down the center vents.