Brief 'Chirp' under hood on cold startup 3L

My 98 Ranger 3L v6 (manual tr, ac, 40k mi) chirps and squeaks under the hood on cold startup. The noise lasts only a few seconds and stops. There is no further effect; the vehicle runs fine. The problem has existed for several months and is very slowly getting worse. First mechanic said it was the vacuum boost, and I agreed. Replacing that made no change. My next guess was the belt which I replaced, again no change.

Today I had my wife start it cold several times while I listened and it sounds like it is coming from the top of the block/head area.

Any ideas?

Take off the belt and check the idler pully

Not that kind of chirp--I recognize that noise and I checked it when I changed the belt, but thanks for the suggestion.

This sounds more like two squirrels having a fight for a second or two.

Different Ford, Same noise...I searched for days trying to find the noise. turned out to be the gear on the starter. when it disengaged the gear spins on the shaft. Dry shaft, caused a squeal for just about 3 seconds, put a little VERY little grease on the shaft after cleaning and no more squeal.
Hope this helps

if it's after startup, it's more than likely the alternator. mine would sing a melody before i broke down and tore everything apart, all pulleys, all tensioners, everything. put back together only to hear it again. took off alternator and sprayed some wd40 on the armature (at both ends and under voltage regulator) no more singing, no more headache. mine was a 3.0L also

i had a chirping noise when idling and running. had the cam sensor and cam synchronizer fixed. has not made a similar noise since.