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i have a 2013 ls3 automatic that has a virbration when in cruise control. when the car reaches the set speed there is a short vibration in the driveline. dealer checked car and told me this is normal as transmission is holding back car as to not pick up speed. not sure i believe this and wondered if anyone had the same problem.
You might be feeling the torque converter locking up, it feels almost like shifting into a higher gear.
Also, on our 2009 LS3 A6 (similar setup to yours), if I ease off the power at around 60 mph there is a slight shudder or stutter felt in the driveline. Ours has been doing it since new (now at 59k miles), it never got worse or better.
Many people, but not all, report this with the LS3/A6 combo. Doesn't seem to cause any problems.
When the converter locks up, getting a slight vibration is normal. When using CC to accelerate to a set speed, it's likely that the converter is unlocked during that phase, but will lock itself at the point the speed is attained. Apparently the dealer either didn't know or didn't think you'd understand the more technical explanation. I'd bet on the former.