Engine shudder in 08 LTZ w/ 3.6

I have an 08 LTZ 3.6 that's having a mild shudder while idling. The best way I know to describe it is it's as if the engine is out of balance, if that can even happen. It's not a vibration, but "soft" shudder. It is worse when idling in "D"rive, such as at a stop light, than in "P"ark, but isn't at all noticeable during acceleration or cruising. There is no drop in RPMs or error messages in the DIC. I have looked through some of the threads here, but none of them seem to encompass my issue. I just bought this car a month ago and have it under the GMPP Major Guard Warranty, but I would like to be armed with a little knowledge of this issue in case the dealer tries to tell me they can't replicate it or claim it's normal. Thanks for the help guys!!!