Not aware of how close the piston runs to the head, but a loose (?) rod could let the piston rap on the head when the oil (and clearance ) changes with higher temp...

I had this with Caddy in the old days, but maybe just a brain fart here....

 

Maybe this will help?

 

Alright so just to clarify we're all assuming this is a rod knock? I'm going to drop the oil pan this weekend and as long as the crank looks good...im going to replace the rod and main bearings along with a new oil pump.

No knock at cold start up. Knock starts when engine is warm. Warm idle oil pressure is 20psi. Driving down the road it sits between 30psi and 40psi. At higher RPM's (north of 3000RPM) the knock gets really loud.

 

I just listened to your video. Did you check the flexplate bolts?
That doesn't sound like bearings to me, it sounds like a broken piston skirt but there's only one way to find out for certain.
If it IS a skirt it isn't too bad to fix.

 

I'm going to start with the flexplate bolts saturday morning. If not them I'll drop the pan and see if one of the piston skirts are broken. If not that then I'm going to pull the bearings and see what they look like.

 

Are you familiar with Plastagauge?

 

Well shoot, too bad you didn't rev it a bit, but I'm with Radi, never heard a rod do that. You have pulled the valve cover an all looks OK up there? Crank pulley looks OK? (timing mark seems right, there is no rubber sticking out, bolt is tight?)....

 

I can always make a new video. I'm actually going to run out to the store real quick. I'll take the jeep to get it warmed up and get another clip.

 

Check the crank pulley bolt before you go....

 

I am currently driving a 99 xj and own a 98 as well. Both vehicles sound just like yours. The inline 6 is a very noisy engine like someone else says. My 98 has over 200k on the engine. I honestly think it is piston slap but a little more intense soundings to the engine wear. Both 98 and 99 are/were DD. Has the vehicle been used for off roading or towing?