I have a 2010 Yukon denali. vortex 6.2l sfi with afm. 73000mi #4 lifter collapsed and was replaced with all other lifters on that side. never knocked before the lifter collapsed. took it right up to the shop when it happened. all rods were checked and rockers. engine is clean and tight. knocks at idle and through gears. knock dissipates at highway speeds and under heavy load. doesn't seem to have any of the sounds of a piston slap or bad bearing. my mechanic seems to think it has something wrong with the solenoid's that allow the afm to deactivate cylinders. oil pressure is good and lifter filter was checked. oil changed 2 times to verify no metal. lifter collapsed very suddenly and had no signs of a problem before. I find It hard to believe the cam was damaged. if anyone is very familiar with this any can offer any incite I would greatly appreciate it.