I have a 225 hp 04 Roadster, with ~ 50k miles.  When I'm stopped at a light with the AC on, the car will emit a harmonic vibration and the idle will fluctuate.  If I turn the AC off, the vibration stops immediately and the idle goes back to normal.  Anyone ever run into this?  Thanks

I get the same thing on my 2003 225 quattro coupe. Turn off the AC and its fine. Let me know if anyone has a note on this. Of course, it never does this when its at the shop.

Dan

My AC emitted a sort of in-n-out "ringing" (but no major idle fluctuations).  When I added a large can of refrigerant, this went away.  Have you checked your AC refrigerant level?  I just picked up a large can with in-line gauge at a parts store (okay...it was a Wal-Mart).  I followed the directions, checked the low-pressure side gauge reading and it was low.  Added the entire can, though it still appears I could add more.  This cured my problem, though I just wanted the AC colder.

Your A/C compressor might be going out.  You may want to have it serviced or checked by the stealer or you can try and add some refrigerant as Patrick suggests.  Those usually contain some

lubricant which may help to lube some of those parts that put strain on your motor to cause the

vibration. 

I had that... blown compressor, about $2000.