My A/C is making a hollering sound. Very high pitched and very irritating. It doesn't happen all the time, but only every now and then. I thought it was something stuck in the fan, but I noticed that it only happens when air is blowing through the upper vents. If it's on the floor vents or defrost, it doesn't do it. Any ideas as to what to check for? Could something still be stuck in the fan?

 

If the noise is inside the cab and NOT under the hood, the blower could be on its way out. Another possibilty is that there could be blockage of some sort (leaves, critters , etc.) in the ductwork that's making the blower work harder and producing noises. Pull the blower down (only 3 screws although the back one is a booger to reach) and test it out of the system.
WC