How to determine if a ceiling fan has a DC motor?
Fans from big box stores like Home Depot and Lowe's tend to be cheaper because they are made from lower quality materials.  Using DC motors in ceiling fans is still relatively new and most of the major fan manufacturers only have a few models that are equipped with DC motors.  The major benefit of using a DC motor in a ceiling fan is lower energy consumption, NOT reduced noise level.

You can expect to pay at least $450 for a ceiling fan with a DC motor.

I suspect 1 of 3 reasons for your humming Hampton Bay ceiling fans:


Are you using a switch on the wall to adjust the speed of your ceiling fan?  If you are using a light dimmer (or any control that is infinitely variable), your fan will often make a humming noise.  You need to use a "stepped" speed control, something that clicks in 3-4 specific speeds.
The fans you own are cheap.  Visit a ceiling fan specialty store and purchase a better fan.  You'll pay more, but you will get a higher quality fan that runs quieter.
Very rare, but the architecture of your house can amplify the noise of a ceiling fan.  If your fan is mounted on a wooden beam, fans with strong motors sometimes generate a humming noise.  Possible solutions: purchase a cheaper, weaker fan OR purchase a more expensive fan, preferably and Emerson fan with a K55 motor and a flywheel or a Casablanca fan with an XLP motor and flywheel.