Back in Feburary 2004 I had the same problem with my 97 Mountaineer. I
described it as a chirping and it would come and go and was a "bear" to
figure out. I changed all the things that other have posted and was
getting ready to change the water pump out of frustration. I will bet good
money that  the noise is coming where the old fashion ignition distributor
was. It has been replaced by a Cam Sensor Positioner which is a long metal
shaft that goes into the engine hole where the distributor use to sit. On a
v8 it is in the front of the engine, and A v6 it's in the rear. On top of
the cam sensor postioner is a plastic piece which is mounted on top of the
cam postioner shaft. This plastic piece is infamous for having a little
magnet inside the housing that breaks loose and grinds against the
revolving metal shaft.This makes the weirdest noise including chirping and
a loud "howling" sound sometimes. If you need more information on how to
replace this piece or questions, please let me know. It's only about
$35.00 bucks but will cost you hundreds of dollars if a dealer does it.
Good luck!