I'm not sure what's going on but it might be a collapsed lifter or does it has active field management on the truck. So I have purchased a kit that will get rid of the active field management but I have not installed it yet. I am waiting. I have now where to work on the truck so until then I have to drive the truck like it is. GMC is telling me that this is not an issue but they're full of it. They know it is. It's been doing it for years on a lot of the trucks. Even some of the newer ones leave it to GMC to screw it up and then not fix it..