[Leica] why do bombs whistle?

[neal at photoneal.com (Neal Friedenthal)]

One of the reason that bombs whistle is the fuse.  The bomb has a fuse 
that is in a safe position while in the plane, as the bomb is released 
the safety pin is pulled and a propeller like device on the fuse spins 
to arm the bomb.  This propeller causes much of the whistle, as the bomb 
accelerates during its fall, the pitch increases.  Modern dumb bombs use 
fuses that lack the external "propeller". 
Artillery projectiles make more of a shrieking sound due primarily to 
the shape of the projectile and their tendency to wobble in flight, flat 
based projectiles tend to wobble more than "boat tailed" or fin 
stabilized  projectiles and tend the shriek more and louder.