Variable pitch whistling sound that changes with engine speed. Not consistent. Sometimes accelerating quiets it other times decelerating calms it. Happens with A/C on AND off. Changing serpentine belt did not work. If ATF were low could tranny be causing this? Belt tensioner? Alternator bearings? When car has been 15+ miles the sound goes away.
My guess without seeing it would be a belt tensioner. It could also be a water pump, power steering pump, or anything else attached to the accessory belts.
Click and Clack recommend making a " car stethoscope" out of a length of hose and a funnel so that you could determine exactly where the noise originates.
Are you the Blair Thomas from Penn State?
Whistling sound suggests a vacuum leak. Check all vacuum hoses and connections.