I have had a noise too... Mine is in for front differentials pierced the housing and now it all has to be replaced. Also...I noticed my gas mileage hasn't been that good at all.....so I called my service guy and since they have it already to check that out for me too ! good thing I read this post, may save me a trip to the shop! and I have that same noise but only on a dirt road......
Thanks Guys !

 

Updatefrom GM and the dealer---
I will be issued a 6 yr./75,000 mi. bumper to bumper warranty.
The greatest asset is to have the service manager on your side.He has the "power" to make it happen.