It's impossible to diagnose things like that over a computer but to start the process, where do you feel the vibration, the seat, steering wheel, or entire car? If the steering wheel shakes, a worn inner cv joint housing is somewhat common. They will cause the steering wheel to vibrate when under load as in accelerating, and it will go away when cruising and coasting.
Vibrations felt in the seat are often due to a broken belt on a rear tire. Bent wheels will do that too. Broken tire belts that aren't real bad yet often are more noticeable on high-speed corners like on highway off-ramps.
If you feel the vibration when the car isn't even moving, that leaves out everything related to wheels, tires, and drive train. There's a misfire in the engine, the outer cast ring on the vibration damper has turned and is coming apart, or, on some engines, the balance shaft is out-of-time. That balance shaft doesn't usually get out-of-time on its own. That happens from being assembled improperly.
Saturday, March 24th, 2012 AT 5:19 PM