[Flatrater]

I can say this since I work in a dealership. They lied to you unless you have a rear diff service somewhere else. GM has a known problem with this and the first step is to replace the diff fluid. ZI have replaced several rear diffs on this model. The biggest problem causing this noise ishavinf 4 tires that are not worn even. If you have 2 new tires and 2 used tires the circumfrence difference will bind the rear diff till it explodes. Make sure you change all 4 tires at the same time and rotate them regularly.