Squealing noise help!

My Buick has a squealing noise when its idling. I can't seem to pinpoint the source since everything is tightly packed into one small area. I did change the idler pulley this past weekend thinking that was the culprit. It did quiet the noise a little, but it's still there.

The noise does seem to go away when the engine is reved up a little. Does anyone have a clue as to what this might be? Could it be the drivebelt, or should I look at the alternator or waterpump?

Thanks for any and all suggestions. This noise is driving me nuts.

Gary