Hammering / knocking noise on 4.0L6 '99 Cherokee

I would say, I have a severe case of piston slap, but from what I have
found out about the infamouspiston slap problem, it should have ended
long before 1999.
So, what problem do I have on my '99 Cherokee? The symptoms are very
similar to the legendary piston slap:
Symptoms vary from no noise, to a light ticking, to a
ticking/hammering to a
strong and very audible hammering/knocking noise. The noise is
generally more pronounced at start up, when the engine is cold.
Sometimes, the noise disappears when the engine warms up.
Sometimes, when the noise appears at start up, stopping and
immediately restarting the engine stops the noise completely.
The symptom is most noticeable during idle, but will get louder as
engine speed increases, and sometimes appears / disappears randomly on
a warm engine and at various driving conditions.
While this noise was very infrequent before, now it is a almost daily
occurrence.

The engine has about 60K and was serviced regularly at the local
dealer, with
oil and Mopar filter changes every 3,000 Miles. The engine runs,
starts,
and idles otherwise completely fine. No significant oil consumption
either.

The dealer here in Thailand suggests to open up the engine, and
perhaps change the hydraulic lifter, but he is basically not really
sure, what causes the problem.

I have used a Fuel Injector Cleaner without success.

Most likely, it is not the piston slap problem, as the car is built in
1999.
If it is the lifter, why stopping and restarting the engine clears the
noise
immediately sometimes, but not all the time.
The noise appears at a cold engine and on a warm engine as well, and
during idle but during normal driving conditions as well, but
randomly, it seems. If drive at high rpm, there is generally no noise.

What could cause that noise?

Any help from you old timers and experts would be greatly appreciated.


--------------------------------------------------------------------------------