weird knock at idle?

i have an 90 xj 4.0 with a knock at idle only. i goes away when i bring the rpms up. its only at idle when the engine hot. its still there when i put it in gear. so its not the flex plate. i replaced the rod bearings and oil pump and its still there. i have good oil pressure. no noise from the top end,so i dont think its a lifter. it also has a new timming chain. i'm stumped? it has great power and runs good. has anyone else had this problem and figured it out? thanks mike.

It's most likely piston slap. Probably 75% of the 4.0s out there have it. Nothing to worry about.

I've noticed that too, what exactly is a piston slap?

Could it not still be the Flex Plate I belive I have the same sound and thought that is what it was.

Am I correct does the flex plate still spin when in gear.

I notice the noise once it warms up but still comes and goes.
oil psi drops from 40 to about 30 when it is making nosie, was thinking becase of the rear mani seal which is leaking some.but when noise quits psi right back up to 40-50.Take the RPM up to 1800 and noise is gone.

Sound like the same thing?

my 4.0 seems to be getting louder by the day..she wants to become a diesel one day..lol

pete