Hey guys, My 1996 Rav4 makes a loud vibrating noise at 4000 RPM. it is always the same RPM and the vibration then goes away but the pitch of the engine is very different after that. After 4000 RPM it is much more like an out of control cheese grater than anything else. Could this be a harmonic in some loose piece of metal somwhere in the exaust causing crazy vibration? I am pretty sure it is not comming fron under the hood as when I rev the car with the hood opened and i am looking under the hood, the noise seems to come from farther back. the initial noise at 4000 rpm sounds alot like when you take a piece of sheet metal and shake it, but much faster.