Rear passenger tire went out of balance to such a degree that the entire car was rattling, mirrors shaking, at highway speeds. Vibration visible to observers outside and following the vehicle. After multiple attempts to balance the mechanic removed the tire from the wheel and found that the "quite running foam" had separated from the inside of the tire. Removing foam solved the issue. About a month later front passenger tire went out of balance, again, noticeable at highway speeds. Not willing to risk being stranded away from home, I have now personally paid, with some relief from GM, to have all four tires replaced with non-quiet running tires. I did a quick search, and if you look at the first comment in this "how to repair" article you will see others have had this problem too. Https://GM-techlink.com/?P=8677

- Grand Blanc, MI, USA