My Dakota is making a chirping noise on deceleration and at idle. It pretty much only does it when the throttle blades are closed. Noise seems to be coming from the area of the A/C compressor. Only does it after the truck is good and warmed up. It will make a whistle when the key is shut off, and will sometimes whislte after the motor has quit turning. I am inclined to think vacuum, but I have checked every hose, and gone all around the throttle body with WD-40 and can't find a leak.

Any clues?

Thanks!