During the winter time when I have my heat on, my outdoor air conditioner unit randomly makes this awful-sounding squealing/screeching sound. It lasts for about 15-30 seconds at a time, stops, and then occurs randomly again.
A Google search has revealed other people complaining about a humming noise, but this is anything but a hum.
It doesn't happen when I simply have the cool air on. It's only when I am using heat.