I turned on the A/C the other morning as usual then pressed the "inside air" recycle button to help cool the interior down faster when a thumping noise started coming from behind the glove box. I pressed the "outside air" button and the noise stopped. I turned off the A/C but the noise kept on. Only when I pressed the "outside air" button did the thumping noise stop. Any idea what this might be?

Thanks for all info!!!
Mike