[theholycow]

Found this:
http://www.rockauto.com/Newsletter/archives/9-5-13.html

Diagnose Intermittent Belt Chirping or Continuous Belt Squealing
V-belts are forgiving. The A/C compressor suddenly seized up on a very rusty but loved ’68 Chrysler 300 I drove for many years. No problem, I just pulled a pulley roughly the same diameter and offset out of my brother’s junk box. The car was quickly back on the road and the belt never made a peep.

Modern serpentine belts are more finicky and will likely intermittently chirp or squeal in protest if something is not quite right. One way to try to track down the cause of the noise is to carefully spray a little water on the ribbed side of the serpentine or accessory belt while the engine is running. If the chirping noise stops until the belt dries out, then the noise is likely caused by misaligned pulleys, wobbling pulleys or damaged belt ribs.

If the water lubrication just makes the squealing noise louder, then the belt is probably tensioned incorrectly and slipping. Replace the belt tensioner(s) or adjust the tension.

If water on the belt has no effect, then the noise could be coming from a bad bearing in one of the belt driven accessories (alternator, A/C compressor, power steering pump, etc.) or the belt tensioner(s). Use an automotive stethoscope (found here under Tools & Universal Parts) to help pinpoint the source of the noise.