Why do airplanes make a loud screaming noise when they go down and are about to crash?

This does happen, but not every time like Hollywood have us believing. The loud scream is caused by either the propeller tips going "Super Sonic," or a portion of the plane does the same thing. The Idea that this happens every time goes back to the WW2 Ju 87 StuKa dive bomber that was fitted with a siren to increase the fear of the people being attacked.