I would just like to follow up here..
This issue on the L99 (automatic cars) is according to GM said to be a result of the AFM (active fuel management). I was able to get them to find something in their system about this. The bulletin no. is #06-06-05-001E.
I am sorry I am unable to post it but maybe a google search will turn this up?
Anyhow, it explains how vehicles with the V8 auto will experience a roughness, rough ride, vibration in seat, car or rear view mirror when this happens. May also be felt in the floor pan, gas pedal or steering wheel. There is also a notable change in the exhaust note when the AFM kicks in and drops it down to 4cyl mode.
Outcome is the good 'ol this is normal and there is nothing to be worried about. They're not addressing this with any type of fix. This to me in BS! Like I am happy that I was able to find out this is the AFM system and not a tranny issue or something but, really!? How do other manufactures find a way to make this vibration non-existent in their products ? Apparently this is not noticed in the Silverado for example with the 5.3L which also uses AFM technology to save fuel.
Point being is it's pretty ridiculous, especially since I really love the car and the thrill of driving it. When this vibration kicks in I feel it up my spine (so to speak) and it really does take a lot away from the driving experience. It's really too bad.
I hope this helps some of you who are noticing this in terms of having an answer on what it was you've been experiencing.