This is definitely not a tire vibration. It is almost like a slight howling vibration. The best explanation is, It is very fast, close together, and low noise. It does not matter the speed. If I stay at exactly 1600 rpm i feel it in the floor board under my feet and only at 1600 rpm. i don't feel it in the steering wheel or in my seat. It is driving me nuts because my truck runs 1600 rpm at 60 mph. When I run below or above 1600 rpm, let off the gas, or accelerate, it goes away. Outside of that the truck does not show any signs of anything wrong and runs great.