Could someone please tell me how to fix this problem. I have a 1994 Chevrolet 1500 4X4 Pickup that has a thumping noise when I turn the ignition switch on or off. It will also make this noise when ever I turn on or off the A/C. The noise is located below the glove box. It looks like a flap for the A/C or Heater vent. I can put my hand on this flap and the noise changes. As I said this is a 1994 Chevrolet 1500 Series Pickup with extended cab, 350 V8 engine, automatic transmission 4X4. Any help would be much appreciated.

 

my dads truck did this for a year! and we couldnt find out where ti was coming from.. we left it alone and it finally quit.. i always thought it was the flap for if u put the air on the floor or on the windshield.