i had the same problem with mine! it felt like a vibration when i would drive it was the exhast hitting teh frame when the motor torqued over i took a torch and pry bar and a lil elbow grease and mines fine nowThats what I thought I took care of.
There doesn't seem to be any metal on metal rubbing....from the sound/feel of it, its barely vibrating up against something.
I'll take a look when i leave work. I dno't have much patience lately. If this thing knows whats good for it....it will self heal!