Originally Posted by seamuis

There are a number of different freehub mechanisms and each one has its own distinct sound. A freehub is built into a hub shell (for use of a cassette) and a freewheel is threaded onto a hub. Classic freehubs and freewheels typically both use pawl/tooth ratchet system. A 3spd is typically an internally geared system and is completely different. They typically make a distinct sound when coasting (planetary gear system) For the classic pawl/tooth mechanism, the number of teeth, the number of pawls, as well as the angle of the teeth, all affect the sound. More teeth and and pawls, the louder it will typically be. BMX freewheels typically have more teeth and pawls for faster and more precise engagement, and are often much louder than road freewheels and freehubs. Shimano freewheels and freehubs typically have only two pawls and fewer teeth, so they are considerably quieter.

heres a good video covering some of the most common freehub mechanisms:
https://m.youtube.com/watch?v=syG-5gun1Dw