so when I open up the grill to change the filter for my A/C, I see to air ducts. A big one, and a small one. I'm assuming the big one is for the heat/AC and the small one is the 'air freshner/air cycler.'
The smaller air duct is closed by a shutter/damper. every often, the aircycyler would operate and the shutter would open up. For some reason, it opens up halfway, and then slams shut to make a big thumping sound.