2000 Jeep TJ engine knock

I have a 2000 Wrangler with a 4.0 5spd 133000 miles. I’ve had this knock in the engine that seems worse when it’s cold out but even in warmer weather it starts out louder then after maybe a quarter mile it gets quieter but it’s still there. Sounds like a hard knock, not like a lifter noise. I saw once on a Jeep forum something about a piston skirt knock. I’ve had it in twice to be looked at and they cant find it. We think it’s coming from down below but so far have not been able to chase it down or do anything to change it by using any kind of additives. I’m hoping someone here has dealt with this before and can offer any advice. Thank you