You Learn Something Every Day (if you're lucky) -- Stuka Dive Bombers

Almost everyone has seen a WWII movie where the Allies come under attack by enemy dive bombers, typically German Junkers Ju 87s, or Stukas.

Whenever the bombers would attack they would wail as they dove, sounding something like this.

Previously I thought the sound was due to the steep angle of the planes' descent. Hollywood added to my perception by using this sound for any plane in a deep descent, especially one about to crash.

However, I read recently that the Stuka's wail was very deliberate in nature--some of the models had sirens built in to the landing gear to increase the psychological warfare aspect of the attack.

Not all the Stukas had this "feature," as it increased the aircraft's drag and decreased it's maneuverability. It's still interesting stuff.

Read more about the Stuka on Wikipedia.