It happens when I let off the gas or if I'm just resting my foot on it but not pushing it. As soon as I give it gas it goes away instantly. The vibration in the gas pedal isn't as bad as the grinding/vibration noise itself. The faster I go the more noticeable it gets.
I have a 2012 Chevy Silverado 1500 Z71 with a 6" lift kit and 35's on it. I'm hearing a grinding/vibration noise and feeling vibration in the gas pedal and steering wheel when I let off the gas. It only starts being noticeable when you hit about 40+ mph but it gets even worse at 65-75 mph. I've had the lift kit on for about a year before the problem started.