For a couple of years (yeah, years) my 2000 JGCL WJ 4.0 had a high pitched whine/whistle, but only when cold, when a bit above freezing and lower, just before upshifting, and then it would stop, and then the shift would complete. Once the engine warmed up it didn't happen.
Finally asked my neighbor and he found that it was the idler pulley. (He found it by listening with an engine stethoscope I have, held against a connected but non-moving part. (Center shaft of the pulley which the pulley rotates about.) He could hear sounds from the bearings.)
I got a new pulley and after I had the old one off you can plainly hear the grinding as you rotate the pulley about the center.
For diagnosis, use a metal shaft - stop at the hardware store and get a 1cm (1/2 inch) round bar a foot or two long and put it on the center of the pulley and put your ear against it to hear if that's the problem. Try the other pulleys too.