Doh! I forgot to say which motor mount it was that they replaced -- it is the dead-center front mount. Free play/engine movement was in excess of 2 inches according to the service writer -- should be less than 1 inch.

The vibration for the stiffer chassis is sometimes due to tires that roll with a different amount of crush on one side or the other. Even though they are balanced on a spin balancer, under weight they don't roll quite round. This slight up and down movement has an impact on the chassis. The lighter suspension parts (aluminum alloy?) react more than older parts that were heavier react to the slight up and down movement.

Balance the wheels on a Hunter 9700 Road Force balancer with a technician who knows what they're doing (someone'* dealer may have had them who posted earlier).
That resolves the out of roll rolling unless the amount of mass goes above 12 lbs or so. That'* what they easily got replacing the tire with Michelins. Other brands sometimes have a much greater amount of crush runout.

GM did not discontinue the chassis because of problems with the tire companies not making high enough quality tires. Someone earlier said they had discontinued the H bodies for that reason!!!

Thanks for the postings everyone.

Had the wheels balanced and problem is gone.

I agree, the Bonneville wasn't discontinued for any reason other than they were not selling. Don't get me wrong, I love my 00 SSEi and that'* why I bought the 05 GXP, but there were not enough of us I guess.

Hey guys, I couldn't help but comment here as I have had some of these exact issues with my '05 GXP as well. Vibration present from day 1 of ownership. I have a very good relationship with the local dealer and have been dealing with this for a month or two now.

First trip in the dealer did a road-force balance and determined three of the four Goodyears were out of round. Dealer ordered me three new tires and they were replaced last week. The minute I picked the car up I took it up on the freeway for a spin and knew it wasn't completely right. The vibration had changed, but not gone away all together.

I got it back into the shop today. The dealer re-balanced all the wheels and ordered the fourth tire (doh). According to what they told me, road-force balancing will only check for the tire shape, or roundness. If a tire is slightly egg-shaped, road-force shows that. Balancing with weights is also difficult on these GXP wheels as the weights have to adhear to the inside of the wheel, not be pounded on the outside wheel lip.

Long story short: have your tires road-forced balanced to test the roundness. Even if that means going directly to the Goodyear dealer for the proper equipment. I will post the final results here when they have replaced my last tire.

Cactus

Cactus, can you please put your year and trim level in your signature?

Iwas having the same problem with my 05 and they changed my rear wheel hub assembly. Because it was out of round.

It fixed the problem. They also put new tires on too.

The End of the Line

Update on this issue: fourth tire was replaced and the vibration/harmonic noise was much improved, but not gone.

GM TSB is out regarding this issue and suggests checking the rear wheel alignment. That was done today at my local dealership.

The final answer from GM is "problem is within technical specifications" your problem sucker! I have never been so frustrated with a new vehicle as I am right now.

Bottom line: this car will vibrate/hum/rattle/shake at highway speeds and GM is not going to do a damn thing about it. I am currently looking for other solutions i.e. arbitration.

Completely Steamed in Arizona

I feel it and think it'* the light rims and low profile tires, does it only happen on GXP'*???
I figure :burn rubber off front/put rear on front then put 60'* on rear and burn offfront again then........ wont take long