An update for any interested. After speaking to all the other GM dealers within 50 miles, none wanted to work on the car. After speaking to GM Customer assistance, I think I determined why. If the issue was due to a mistake by the dealership - it's not covered by GM - the dealership has to cover it. So basically the truck had to go back to the dealer who did the work the first time.
So, went to the dealer on Jan 5 to ask if they want me to drive it in or if they wanted to tow it. After a bit of a run around, speak to the GM, and he sends tech and service advisor to my house (I live ~2 miles from dealer) - tech has me start the car, and immediately tells me to shut it off. Truck gets towed to dealership, and they can't get to it until this week.
Long story short - allegedly it was a collapsed #3 lifter and now my truck has all new lifters and associated hardware on the other side of the engine too. That means all new lifters on both sides at 31K miles. They also replaced the drive belt for some reason. All this work was covered by warranty. Only the initial injector replacement based on AFIT results that I dispute was not covered.
Got truck back today, and so far the 2 mile drive home in cold weather and it seems okay - no weird noises or tapping. Time will tell. I just hope it stays fixed.