You're kidding right? OK, I'll bite. I have the exact same model. During the
summer, when pulling up a hill on warm days, I get that sound too. What it
is is pre-ignition or spark knock. A little is normal and nothing to worry
about. If it becomes very loud, causes the truck to vibrate noticeably or if
it occurs when cruising on level ground, it needs to be looked at.  Go to
the local Costco/Sam's Club and pick up a 4 pack of Chevron's Fuel Injector
Cleaner.  Here in the PRK it will run you $9.00 and change for four bottles
verses $10.00 for one bottle at your local auto parts store. Follow the
directions on the bottle and see if things improve slightly. Odds are they
will. At 3+ years since purchase you might have maybe 40K on the clock
already. I know I have just shy of 48K. There is bound to be some carbon
build up in the motor by now, especially on the plugs. This stuff will clean
it out pretty well.

am guessing htat it's engine knocking you did not say what altituse you
were at or how steep a hill you were climbing.....

Sometimes a sound like that can be caused by the starter motor not
retracting.  The gear on the starter motor hits the flywheel and usually
only while heading uphill (on Rangers and Explorers anyway).

What I did to isolate the problem (and verify) on my Explorer was to park on
a hill headed up so that it was making the noise and very carefully backed
out the starter motor while it was running.  Be careful if you decide to do
this.  Block your wheels etc and take car not to short anything out.

BTW mine sounded like it was coming from inside the front of the engine.

nd-ftopict135507.html

What you are describing sounds like the fuel injectors pulsing, which on my
1991 Ranger with the same engine, sounds like valve tapping.  I've been told
by many people that the injector pulse is noticeably louder on these engines
and not to confuse it with the valvetrain.  However, as you suggested with
your own truck, my trucks noise is much more noticeable while accelerating
up a hill or under load.

Chances are, this is probably what you are experiencing too.  I've had my
truck 3 years, it has over 230,000 kms and the engine still runs like a top
and absolutely no engine work. If I were you, I would not let it worry you.

Sharky