I had the exact same problem! I took all the belts off and looked for play and spun them to feel for resistance/listen for noise. My idler had a little play so I replaced it. My alternator bearing was making a little noise when I turned it by hand so I replaced that (think that was my noise at cold idle). I also replaced the AC belt and tightened it so spec (pretty damn tight).

No issues now! I used to be able to run the AC sometimes at low loads but climbing a hill or giving it has caused it to make the noise real bad. Think the belt was slipping. Think the alternator was making the squeaking noise at idle.

So bottom line, turn them all by hand and check the tension.