Sorry about the Exp. post in the forum, but no one seems to look at the Exp. forum much. When I turn on the rear AC in my 04 Exp. there is a knocking noise that comes from the rear pass wheelwell/rear cargo area. The air still works, it just constant goes thump, thump, thump...any ideas?? thanks.

 

The temperature blend door motor is broken. The gears are stripped out in it, hence the "pop" you're hearing.