Having AFM turned off in a tune won't rule out lifter failure. Even non-AFM lifters will fail, as will AFM lifters - even if not switching between modes. But it does also raise the possibility that yours has had more than a tune done, possibly including aftermarket lifters and/or cam.
All I can offer are the steps I would take if I were in your shoes. I should mention that I'm a survivor of 2 cam and lifter failures on my 2012 XL Denali due to bad lifter trays received from a questionable supplier, installed by me during a cam swap. Many of the others advising you here have considerable valvetrain experience on these trucks.
What I would do next:
1/ Take the main accessory belt off and run the engine without it (briefly) to determine if the noise is still there. This will rule out the water pump, PS pump, alternator and tensioners on the accessory drive. Put the belt back on.
2/ Take the stethoscope to the rear of the drivers side cylinder head to rule out any exhaust leak from the (possibly) missing exhaust manifold bolt I mentioned earlier.
3/ Check all 4 spark plugs on the drivers side for tightness.
4/ Follow the procedure outlined in the service manual to safely remove the rocker arms, springs and pushrods on the driver's side without damaging the threads on the towers. You have to rotate the crankshaft in increments, starting with top dead center on cylinder #1 to make sure there is no pressure on the rocker arms as you move sequentially, or you risk stripping out the last 2-3 (aluminum) threads on each tower. Carefully lay them out on a clean towel on the workbench, in order. Inspect each spring for cracks, foreign objects or other damage. Roll the pushrods across a clean, flat surface to look for any bowing. Check the trunnion bearing on each rocker arm, looking for excessive radial or lateral play. A tiny amount is normal.
5/ If everything checks out, move on to the passenger side and repeat steps 3-4.
6/ If everything still checks out, then I would try the Berryman B12 + oil change suggested by Chris. You can also do this step before any of the others because it's easy.
7/ If still no joy, it will be time for the drivers side head to come off, followed by the lifters on that bank. Then the pax side head and lifters if still no joy. It's not a difficult job, but it is time consuming.
