The reason was- some bombs were equipped with whistles! ... When the bomb is dropped from the airplane, the velocity of the bomb goes on increasing due to gravity until it reaches its terminal velocity. As the bomb approaches towards the ground, the pitch increases and thus the “kooouuuueeee” sound.

Do falling objects make a whistle sound?

Whenever an object is falling from the sky towards a character, it is often accompanied by a high pitched whistling tone that both increases in volume and decreases in pitch as the object approaches the ground.

Why do shells whistle?

The shape of seashells just happens to make them great amplifiers of ambient noise. Any air that makes its way into a shell's cavity gets bounced around by its hard, curved inner surfaces. The resonating air produces sound. The pitch of the sound depends on the size of the shell.