When I brought my 05 with MRC into the garage after going through a car wash, I heard a hissing sound like a tire going flat. I quickly removed the right rear tire, but it was ok, and the noise was coming from the suspension. I put dish soap around the air bag and fitting and did not see bubbles. After a few minutes the noise stopped and I have not heard it again. The dealer said I have a bad hydraulic seal in the shock and they want $2200 for the pair and labor. I don't believe them. I am confident it is the leveling system. Perhaps the blast from the underbody flush loosened something up. Any recomendations for diagnosing this? If I do need shocks, I will buy them from gmpartsdirect for $444 each and either install them myself or pay somebody else. Has anyone ever attempted this?