This has been solved, so I'm leaving it up to help educate others.
The oil pan was removed and confirmed that the #6 piston skirt was broken off in a few pieces, #5 piston already has cracks formed. Beyond that, I didn't look at any of the others, no longer necessary for diagnostic purposes.
1996 Grand Cherokee Limited 4.0L (230k miles on original motor) - Similar videos have indicated piston slap or wrist pin. Other possibilities are valve noise, exhaust leak or flex plate.
Some of the diagnoses I've done until this point:
1) Preliminary visual inspection (ruled out known exhaust leak as that's a higher-pitched, faster tick noise - different from this low, slow knock
2) Video to try and locate and evaluate type of noise (sounds louder next to distributor side and underneath near oil pan)
3) Removed flex plate inspection cover and rotated engine (didn't find any pieces missing, hairline cracks or loose/missing bolts)
4) Ran vehicle to warm engine, drove (noise does not dissipate over temperature ranges, stays fairly consistent)
5) Removed injector wire on each cylinder and started engine each time (there was no reduction in noise except on cylinder #6, closest to the firewall - reduction in noise would indicate problem is probably isolated to this cylinder)
6) Removed and inspected spark plugs (all have fairly normal wear with some brown fouling - however, #6 is much darker)
7) Warmed engine and performed dry compression test (results: #1=150psi, #2=140psi, #3=130psi, #4=150psi, #5=140psi, #6=165psi) (I have a photo available for inspection, interesting to note that the lighter to darker fouling on the plugs is also consistent with the compression readings from lowest to highest. 130 is the lightest, the two 140s are about the same color, the two 150s are about the same color and the 165 is the darkest. Higher compression = hotter combustion?) (Another thought, the higher compression and noise from #6 could be carbon build-up - causing higher compression and pre-detonation knock)
8) Oil pressure generally stays around 40psi at idle and fluctuates between that and 50-60psi when at higher RPM. Seems all good and fine.
9) Leak down test... (I didn't experience any significant leak down on any of the cylinders)
![](https://i.ytimg.com/vi/_U7XqHeSr8Q/mqdefault.jpg)