I've noticed that some bikes make a loud buzzing/whirring sound as the freewheel spins, and other bikes don't make anything more than a tiny little purring sound, nearly inaudible. I used to think that the buzzing was a sign of a crappy freewheel, but lately I've noticed it on some pretty fancy $4,000+ bikes.

Is this a characteristic of some brands of cassettes or something? Is it something inherent in the hub, or the freewheel itself?

I for one think that freewheel buzzing would be a terrible joykill for my riding experience, so it would be really nice to avoid products that cause it. I still don't understand how someone can pay $4k for a bike and tolerate that buzzing when so many other bikes are virtually silent as they coast.

Thanks!