My guess is that the high-tech engine mounts that are supposed to help dampen vibration in the 3 cylinder mode aren't working as well as we would like them to work. (That doesn't mean that are defective). The 3 cylinder mode combined with low engine speed (caused by "aggressive upshifting to improve mpg) is a bad combination for vibration. I don't really feel that much through the steering wheel, but it's more like "whole car" or the feeling the engine/drivetrain is vibrating, like lugging down a 4 cylinder manual trans. car.