Hi willi,
Sound travels through different things at different speeds.
The fan blade itself transmits the motor hum through the blade. However, you only hear this hum when the blade is near your ear. Once your ear isn't close to the blade tip, you hear the motor hum thru the air. Since the noise of the hum travels at different speeds thru air and blade material, the constant hum sounds interrupted, like a percussion.
If the air and blade material all transmitted sound at the same speed, you would hear a constant hum from the motor. Since you are hearing the motor through different mediums as the blade spins, you hear a percussion of sound, rather than a constant hum. You see the same effect viewing movement with a strobe light - the movement appears choppy since you only see the movement at different stages, just as you only hear the hum transmitted thru the blades when the blade is right by your ear.
If you were to attach a microphone to the tip of the blade, you would just hear a constant hum through the blade. It is only when you listen to the hum through different substances that you hear the changes in hum frequency.
Does that make sense?
Also, think about how a fan will distort sound waves if you speak through a fan. Also, think of the Doppler effect, how the sound of a passing car changes as it passes you.
