It's actually a sonic boom!
When you snap a whip, that sound you hear is the whip BREAKING THE SOUND BARRIER.
When you use a whip you are creating a wave of movement. The speed of the tip of the whip increases until the end of the wave. The tip makes a snap sound. That means by the time the tip of the whip snaps up it is traveling faster than the speed of sound.
(source)
