Purchased 2008 cts upon driving car home i noticed a vibration. took back to dealer next day. had to wait 3 weeks to get service on vehicle and found out at that time there is a service bulletin on this car and the dealer is not to try to fix car. went to gm and they agreed to change tires but this did not fix the problem, the vibration continued. trying to come to some sort of agreement with gm and after 9 months, nothing. took car back to dealer for tire rotation and requested "road force balance" and the problem got worse. the vibration starts at 40mph and the steering wheel is shaking. the problem is either with the wheels or the tires. *tr