I have been working for six months now to diagnose an idle vibration problem in my 2004 GTO.

I initially purchased a Predator to just bump up the idle speed 100 rpm; this turned out to not work too great. That just made it feel like the "choke was stuck" if you know what I mean, although it did do away with the vibration.

I have gone round and round with the dealer service; they won't fix it and say it is "normal operation." Drove up to visit an old friend of mine in Tulsa; Chris Pipher with Car Analysis Plus. We spent two days on the car; replaced passenger side motor mount; no joy. Spent several hours on tranny mount and exhaust system with no improvement. He was going to check the A/F with tailpipe sniffer, but the O2 sensor needs to be replaced in his sniffer so we couldn't do that.

We are just about to think that it just isn't compensating for the extra load at idle of converter, alternator, A/C compressor, PS pump, etc. Idle vibration is felt throughout the car when in Drive or Reverse and is worse with A/C on. Additionally, it gets considerably worse the hotter the temperatures get. 100+ degree days it gets the whole dash humming and sounds like an old diesel truck with a hole in the muffler.

Could this be tranny/converter related? I have noticed that you can feel a drag or decel when you let off the throttle at highway speeds. This may be normal; I know my 04 Silverado almost feels like it speeds up when you do the same thing; i.e. no drag at all, seems to coast.

Would it hurt anything to temporarily pull off the accessory drive belt to see if that changes anything. I love this car and want to get this figured out; it's really driving me nuts.

BTW, this is not cam shake; I know what cam shake is. This is a constant low-frequency vibration/resonance combination. It can be felt as well as heard like a very low constant bass note.