Rear A/C not blowing air

I have a 2000 Chevrolet Tahoe LT. I'm having problems with my rear air conditioner. When I turn it own, I hear a loud fan sound and no air blows from any of the vents. Has anyone ever had this problem or know where I could start diagnosing to fix the problem. Thanks a lot

There is an air return in the truck somewhere, its probably clogged up. Look for a small grill in the back, probably on the drivers side.