I have a 2007 Chevy Monte Carlo with the 3.5L V6, 106k miles. It has a small, steady vibration that doesn't change with speed or rpms, and although I thought I heard a whistle, it doesn't seem to be making any noise. Someone suggested a bad harmonic balancer, or a loose exhaust pipe. It runs absolutely fine, with no check engine light. A shop I brought it to for a tire press monitor problem put their scanner on it, and could find nothing wrong. They replaced two motor mounts because there was play in them, nothing changed. You feel this light vibration mostly when stopped at a light, in gear with your foot on the brake. Power braking doesn't make it worse. It's been this way over a year, since I changed the spark plugs, wires, and coil pack, and cleaned the mass air flow sensor ( with MAF sensor cleaner) all at the suggestion of a friend. The car gets occasional fuel injector treatments. Any ideas or suggestions? Thanks.

 

You're going to have to define vibration a little better. Is it a noise, a feeling, etc?

 

A feeling. I did just shine a flashlight on the balancer, no wobbling.

 

Is the exhaust system shaking? Perhaps a bad torque converter? Have an assistant sit in the car with foot on brake in gear and you go around and observe all parts of the car.

 

Those engines often have a slight vibration, they're not as smooth as V-8's or inline 6's. Consider sitting in another one to see if it feels the same.

If yours is rougher than others check the motor mounts and transmission mount also.