A/C Problem

I have a 2000 Chevy Venture and the a/c is not working properly. It sometimes blows cold air in front, sometimes not very cold at all. The rear a/c is blowing warm air.

There is a vibration/rattling noise coming from the engine compartment near the a/c compressor. I think it could be the clutch, but then wouldn't the air in the front always blow warm?

Is it possible that this is a problem with the tension pulley?

I have checked the refrigerant pressure and it is ok.

Any ideas?

the clutch might not be engaging all the way or the bearing might be bad. get some one to turn the a/c on the off and see what the clutch is doing.the pulley should be running true no wobble. if it is wobbling then bearing is gone.