I had the same problem on my 2008 until my tire store showed me one of the rims with the tire off. The rims were pitted and corroded on the inside. This was enough to allow air to leak out slowly. The "chrome" on the outside of the wheels was looking bad as well. To my amazement, the dealer replaced all 4 rims at no charge even though I was out of warranty. It is a known issue, so I would try to get the dealer to replace them. Even though I was glad to get new 22"'s, GM farms there stuff to the lowest bidder, so I wouldn't be surprised if the pitting happens again. BTW, I live in Florida so road salt is not the cause of the pitting, it's just cheap wheels.