I know that problem. Sounds exactly like my '97 Limited with 104,000
kms. I looked around the group for files and info and I narrowed it
down to the IAC (idle air control). I can't remember where the info is
but it is here somewhere. The IAC is a flanged device with a wired
connector on the opposite end of the two-bolt flange in the air intake
pipe after the throttle body.

Disconnect the connector (don't force it, it unplugs easy once you
release the lock right) and remove it by loosening the two bolts with a
10 mm spanner of box wrench.

There is also what looks like a black knob on the side of it. Its isn't
a knob. its the housing for the felt wad that serves as some kind of
air filtre. The black outer cylinder comes off with a little persistent
but careful pulling.

Wash off the felt pad with detergent (dishwashing stuff works) and set
it aside to dry.

Set the IAC itself on some kind of metal pan and spray liberally with
carburator cleaner. You should get quite a collection of soot and dirt.
I air dry it with canned air--photographer's stuff. I know, I know,
thats pricey. You can use shop air too but be careful the thing doesnt
get blown away and into the floor.

Spray the bits inside the flanged ports with some wd40 or something
similar and reinstall in reverse order.

You will notice that the problem won't immediately go away albeit a
distinct reduction in the whistling. Drive around a bit and it
eventually goes away. i hope this helps.

Here's some info:
1. While the engine is basically the same, Ford changed the output
slightly--the '98 SOHC engine had 5 more ft-lbs of torque than the
1999/2000 but in exchange those years had 5 (or 10?) more horsepower. I
remember comments at the time that this wasn't a good tradeoff because
the higher torque was more useful than the higher horsepower.
2. Your '98 may have had a higher axle ratio(certainly did if it had the
trailer tow package)
3. The 2000 may weigh more than the '98.

Ken