'97 TJ, 4.0, roughly 130-140k on the odometer.

Bear with me, as I'm trying to narrow down the source of a persistent
engine noise.  I'm well versed in mechanics, but this is the first
engine "rattle" I've run across that has been more noticeable at idle/
deceleration than under a load -- as a matter of fact, it's gone once
you get the slightest load on the engine.  I'd say it's louder than
the constant tap-tap-tap of a bad lifter or rocker, but far softer
than bearing noise.  At idle, it the sound oscillates a bit and isn't
quite as "even" as it is upon deceleration (where it sounds in time
with every cylinder).  Based upon the sound, my thoughts wander toward
piston slap or a really loose timing chain, but that doesn't gel with
the no-noise factor under a load, or with every other worn-out engine
I've dealt with.

I only ask now because, with the recent failure of an idle control
valve, "honey" idled in traffic on the way home at 3,000
rpm......which happens to be right around the no-load resonant
frequency of this sound....and either it's a fair bit louder now or my
head makes me think so.

Just to cover a few bases:

Not transmission/clutch related.
Not an exhaust leak.
35 psi oil pressure at idle, hot on a manual gauge with 10w30.
Runs and idles smooth.
Present hot and cold.
The harmonic dampener not hitting the timing cover.
Sound does not emanate from inside the valve cover.

Hoping you all can shed some light on this issue -- I know this engine
can be dependable and has a few noises that creep up over time, I'd
just hate to miss an opportunity to service an overlooked part during
a maintenance tear-down....or worse yet dive into a sand bowl with a
real soft engine...

Thanks in advance,

Jon