I've got a 2007 G35X, and had a G35S rear sway bar installed last week. After the shop installed the sway bar they did a 4 wheel alignment, it was much needed. Everything felt great, noticeable improvement! Up until last night the car felt great. All of a sudden, this morning on my commute to work I started to get a pretty significant vibration around 60mpg+. It seemed to be coming from the rear.

After noticing the vibration, during lunch I had all wheels balanced and rotated. The vibration is slightly better but still noticable at 65mph+ or so. The shop today said they were fairly out of balance, but the tech didn't notice any vibration on the post-balancing test drive up to 75mph and all suspension components looked good.

I think the vibration gets worse the longer your maintain high speed. Thoughts?

 

I thought a wheel weight may have come off this morning, but after the new balancing I'm still noticing the vibration...

 

Were they roadforce balanced? A "regular" balance is very hard to get a perfect balance. roadforce balancing may cost $10/tire more but it's worth it. Also, you may have bent tires. Most shops will not tell you this or not even investigate this--usually they cannot fix it anyways. Always pays to go to a alignment/balance specialist. Lastly, make sure you take the time to mount the wheels properly--a lot of shops are too lazy to do that even.

 

If the vibration gets worse the longer you drive the vehicle them it could be a rear brake caliper sticking and it will vibrate worse the hotter the rotor gets due to warping.

 

well it sounds like tire balance. Becasue you said it got a little better after the first balance after you found the issue..if it was something else that balance wouldn't have changed anything