slight low end knock on startup, maybe seconds

looking at a new rig, and MJ , (4th one) and heard a slight knock on startup, lasts maybe 5 seconds max, only after sitting after running(oil is thinner, not like sitting cold overnight)

could it be piston slap?
main bearings?
rear main was leaking, also had a bad exhasist leak. doesnt last long and goes away after a bit. he had to redo the head gasket in the last 1000 miles. wondering if the coolant washed out the bearings?
carbon buildup?
spun bearing?
it goes away really fast, but only comes on when the jeep is ran, warmed up, then turned off, sits for a few , then started. niether of my 2 xjs do this.
bad oil pump? oil presure was good on the guage...

If it goes away, it's not bearing related.
Piston slap is common but doesn't go away fast. It reduces more and more as the engine is warmed up.

What brand of oil filter is on it? If it's low quality, they drain out each time the engine is shut off.

sounds like a bad lifter that is bleeding down and when you start the jeep it needs to pump back up.

sidewaysstarion said:

sounds like a bad lifter that is bleeding down and when you start the jeep it needs to pump back up.

Click to expand...

Very well could be. I suspect the same. Still curious about the brand od oil filter though. Marginal lifters don't like being fed air instead of oil on start-up.

When I used fram oil filters my 4.0 would do this.

Mine does the same and i have a purolator on right now

We need to know what filter blondejon has on his Jeep.