Rear AC Thumping Noise

I recently noticed that when I have the AC on that I hear a thumping which seems to be coming from the rear of my Mountineer. Initially I thought it was wheel based but I noticed while idling in the driveway that I could hear it even without the AC on. When I turn the AC on, the THUMPING is more pronounced. Are there any AC devices or motors in the rear, perhaps a motor or something?
Thanks