2003 belt chatter, rattle

I have a 2003 ZTW. 2.0 liter. When the A/C is on, and the car is idling in drive with my foot on the brake (like at a stoplight), there is this awful, loud chattering noise that comes from the lower serpentine belt area. My mechanic is stumped. We replaced the belt tensioner, it had no effect. The mechanic says he has heard other focus' make the same noise. Again, it is only with the A/C on. I have had the compressor replaced too because it failed, so it shouldn't be that. I read on another post about that dual-mass flywheel weight noise. Could that be it? Everything works, it's just embarrassing as hell because it's so loud. I can take the car out of park, or shut off the air, but who wants to do that every time they stop? Suggestions please.

Ask your mechanic to install his gages and monitor suction and discharge pressure. I think you may have trash from the compressor change in the system.