This question asks how to find ceiling fans that don't audibly hum:
It is something that annoys me too. I can't stand the hum a lot of ceiling fan motors make. Our current house has many Hampton Bay fans that hum like crazy.
Over time I want to upgrade these and the answers to the above mentioned question point out that a big part of the hum is due to how AC motors work and that finding fans with DC motors would be a good thing.
How does one do that? Are there particular brands that use DC motors? Or do you just have to look at the box and hope it's in the spec's some where? Is it safe to say that, in general, DC fans would hum a lot less than AC fans? Do more expensive fans tend to use DC motors or is cost not really an indication?