Mine does the same thing -- I have a 1995 XLT, and have owned it from the
beginning.  I've ALWAYS changed the oil every 3000 miles and done everything
that's needed.  It runs perfectly.  A mechanic told me that they do this,
and mine has been doing this at least 5 or 6 years with NO problems [I have
122,000 miles on it].  I'm not a mechanic, and don't know much about cars,
but I wouldn't worry about it.

That's a reliable engine with low miles. Best bet is to take it to an
experienced mechanic with a good ear. Some possibilities besides piston
slap:
* rod bearing knock
* piston pin loose
* hydraulic lifter sticky
* valve adjustment out of range
* low oil pressure causing lifter noise

The valve lash related problems make clicking noises, the connecting
rod noises are louder tapping noises.  You can try holding a piece of
hose to your ear- move the other end around while listening to find the
loudest area. You should be able to tell if it's coming from a rocker
cover (valve lash related) or the crankcase (rod related).

Since it goes away when your rpms come up I'm thinking low oil
pressure. You might change the oil and filter, and examine the bottom
of the drain pan for metal particles. If you see gold colored
particles, you have a bearing going.
-Paul

When I first purchased (new) my 2000 Limited, I heard a similar noise when
engine was cold.  The noise would disappear after 5-10 minutes.  I didn't
worry too much about it-the car ran perfectly.

After about 6K miles, I mentioned it to the dealer.  He verified the noise,
contacted Ford, then replaced the engine (5.0 V8).  I never knew what the
problem was; it was warranty work of course.

A friend had a similar problem with his Eddie Bauer.  Ford also replaced his
engine.

Just bought a 98' Explorer and had some questions regarding some noise
I hear under the hood. The truck starts fine, idles fine everything
works great. When I go to drive off after a minute or so from a fresh
start, the engine makes a clicking or rattling sound in rhythm with the
revolutions of the engine heard only when I accellerate. The noise
seems to eminate from the passenger side under the hood.

I've done some DIY internet research and some things point to "piston
slap," but it doesn't seem to sound like that is described. The noise
remains until the engine is fully warmed up and then I don't hear it
unless I sit idle for a minute or so, but it goes away fast.  Some
friends of mine said it might possibly be too thin of oil for the
weather (It's currently Winter), a valve is sticky, or a rocker is
loose. I have no idea about these things at all.

The truck runs very good and very strong, so whatever it is isn't
causing any sort of performance issues.

The engine is a 302c.i. 5.0 V8 with 62,5k miles.

I appreciate the help.

Leaking exhaust manifold and/or egr tube leaking