I have a 2007 Impala LTZ with the 3.9 3/6 cylinder light/heavy load option.
the car has 63 thou miles on it, an a vibration has developed when in high gear light
load when on the highway which dissapears apon increasing speed or when car
shifts down to standard regular 6 cylinder operation. No one seems to know at this
time what the problem or fix is....
80,000, no issues. There is vibration at low speeds in 3 cyl. mode when slightly loaded. Knowing what it is, I ignore it. Gets better mileage than a friends 09 without the 3/6 cylinder option, but similar to my 06 without it. Wife drives the 06, probably why.