I have a 2007 GMC 1/2 2wd V6 Auto 96000miles . I just bought it last month . It runs and drives great with the exception of a very light vibration at 40mph . If I am driving through slow traffic and not accelerating and not decelerating just more or less coasting at around 40 mph it vibrates in the steering wheel ,seat ,and floor pan, as a harmonic vibration would feel . It comes right out of it and stops vibrating if I accelerate even slightly ,or brake even slightly . This is a very light vibration and sound ,yet still is annoying . It almost feels like I am driving on those ridges cut in the pavemant at the right side of the road when you accidently veer off your driving lane ? Any help would be welcome on this .