High Revs quieted my 4.0?

My 2000 XJs 4.0L was pretty quiet when I got it - the quietest 4.0 I've had.

Over the past few years, it's got pretty chattery. It didn't sound like a broken / cracked flex plate - I checked and the flex plate was not broken and the bolts were not lose.

Hot idle:
https://www.youtube.com/watch?v=Emcfq3BnlzY

Hot restart seems to be the worst:
https://www.youtube.com/watch?v=KYuny4UQM44

To me, it sounds like a broken piston. Which makes sense - it's a 2000, so it is either going to break a piston or crack the head. My plan was (and still is) to run it until it self-ventilates then find a 99 and older engine to swap in.

I was doing some snow wheeling. I have skinny tires. The only way I could go at all was to use a lot of wheel speed. There were a few times where I had it bouncing off the rev limiter - and a few spots where I sustained 4000RPM for a minute or two straight to move.

After that, the engine was whisper quiet. Even on hot restart!

The chatter came back again. So I put it in 1/2 and drive around at 35MPH in 1st gear for a few minutes. Engine was quiet again.

Oil pressure is good. Seems to be too high pitched of a noise to be a rod knock (same pitch as piston?).

But usually high RPM operation causes the pistons to let go if they are broken.

Confused!

Sorry, I was NOT able to watch the youtube videos.

Could it be sticking/gummed up lifters? i.e. valvetrain noise, really high oil pressure from the high rpm in cold weather might been enough force to push past the gum and sticking, but after cool down and contraction, they stuck again?

Another possibility is a spun bearing, but usually a spun bearing does NOT quiet down with high rpm.

Ford had a problem with their redesigned 4.0 knocking in the late 90s. I had a '99 Ranger that did it. Tech told me to drive it around at like 4500. I don't recall the reasoning but it did quiet it down for a bit. Noise would come back and I would do it again. Sold that truck and got an XJ.

gummy lifters seems like a solid answer to me. Just before your next oil change add a quart of Marvel Mystery Oil to the crankcase... change after 300 miles. Good chance that if it is gummed lifters, the MMO will clear it up.

Seafoam would be better used in that situation. Then use a quart of mmo, in place of a quart of oil on your next change.
My friend Chris has a method of "unsticking" lifters. Basically get the vehicle to NOT. Then over a few minutes start by holding the engine at 2k rpm, going up about 500 rpm every minute til about 4k

Synthetic oil will take a while to ungum them, so the suggestions with Seafoam and MMO would work quicker. But synthetic leaves far less deposits and doesn't break down like conventional oil, leaving behind varnish to gum up the lifters. All oil cleans, its just conventional oil will breakdown sometimes faster than it cleans and leave behind deposits. So, switching to synthetic will cause it to slowly clean deposits away, like varnish in the lifters.

https://m.youtube.com/watch?list=PLBC49F10A9C0FBAFB&v=sh151kZXqVo

Might want to check your rocker arms....or see if you have a bent pushrod.(that was the source of his knock)
Im prone to think your noise is a little deeper tone than a lifter.

Whoops, forgot to mention - once I hit high revs , it's quiet at all rpms, idling too.

I ran synthetic since i have had it ... this is the first oil change on conventional ... but it did it while I was still running t6

Have you ever shined a light into the oil filler hole and assessed the sludge?

Rotella T3 does a great job of cleaning out the sludge along with MMO.

Also gotta ask which brand of oil filter you're using.

I usually shine some sort of light in the pan when I do an oil change. Also check inside the valve cover. It's pretty spotless as far as I can tell; there's no sludge buidup that I can see. In the 30K I have had it , I usually have done oil changes every 5K or so miles. I got it at 100K miles so who knows what happened before.

Oil filter has always been a Ford FL1A under my watch.