Hey guys virtually everytime i start my truck and turn on my a/c it sounds like one of my vent doors is either being slammed open or shut. I could just be being paranoid but it does it when i'm driving to. I'm guessing when the a/c compressor turns off and on. I have no idea what the sound is and the dealer has had my truck enough times for a/c work that they will never see my truck for a/c work again. It doesn't seem to have effected my a/c operation but that scares me and i don't want it causing a big a/c problem latter. Is this thunk noise normal or not. Sometimes its louder or quieter but it fluctuates. What could this sound be?