I listened a few times and it doesn't sound like lower end noise,more like valve train.IMHO the mixed metal theory doesn't hold water at all or the motor would make this racket from day 1.Probably tensioner,dirty lifters or even worn cam lobe(s)One other possibility although more common on GM northstars and 6.0 ltr is carbon buildup on the piston heads,it can make one heck of a noise when first started.Auto RX for the lifters and a piston soak through the plug holes overnight may help(followed by a oil change to get rid of anything that seeped down)
