If it is making the noise when the clutch is depressed, there is a good chance the pilot bearing (what you call the idler bearing) is bad. I just had to replace mine and it only had 80K miles on it when I did a complete clutch job. I thought I had bought all quality parts but the pilot bearing must have been China made.
My symptoms were; a strange squeal/rattle just as the clutch was engaging from a stop. This went on for about a year. I let it go because the clutch worked perfectly. One day I started to have difficultly getting it in any gear occasionally and then it would go away. Then one evening I pulled up to a stop sign and put it in first gear. While I was waiting for traffic to clear the Ranger tried to surge forward, then not and then surged again. I battled with shifting all the way home.
This happened on a Friday night so I decided to pull the transmission on Saturday to find out what was going on. I thought the disc was delamination or something. When I removed the clutch, several worn needles (of a needle bearing) fell out. I checked the pilot bearing and very worn needles were laying across the bottom. It was also totally dry. Apparently the needles were so worn they got crossed up in there and jammed against the input shaft that rides in the pilot bearing.
I don't know if that is your problem but if you didn't replace it, it is a good candidate for the problem. Even if your pilot bearing is a bushing bearing, excessive clearance can cause a vibration that sounds kind of like a squeal. Once the clutch is engaged, the pilot bearing is no longer loaded and will not make a noise. If you are hearing a noise with the clutch engaged (out) you probably have a problem with a bearing in the transmission.
2002 FX4 Bright Red Supercab 4.0 SOHC, manual 5-spd & Tcase, 33" BFG MTs, K&N air filter, Modified air box inlet, Gibson SS cat. back exhaust, Bama flash, Hidden Hitch round receiver, Rear helper air springs, Electric trailer brake controller, Line-X bed liner, WAAG bar with KC lights and Wildernest shell.