Yes, the issue was resolved...ended up being the driveshaft, apparently GM knows its a problem, cause the service manager at the dealership called them and they said to replace it. Problem solved!

 

It is a common problem. Was it a one piece D/S? I replaced my u-joints less that 3 months ago, and the knocking is back already. It had literally destroyed the pins in the bearing cups. Some were powder.

 

Yes, it was/is a one piece aluminum driveshaft. They replaced the rear u joint first, it was fine, but still made the noise...

 

Thanks for the update. When I took it in for this, about June/July of this year, they first couldn't reproduce the sound. Then I come over and spent about an hour with them. That seemed to be beneficial for both parties. The click/pop was definitely coming from the drive shaft. They replaced the "drive line" and the problem seemed to be fixed, but that was only short term. It came back. I will take it in some time soon and ask them to replace the drive shaft again.