Some dimmers chop the top of the sine wave off to adjust the power going to the bulb. Rather than being an nice smooth curve, that abrupt off, then reverse causes the filaments to ring, or humm. Most manufacturers do a good job of supporting the filaments so they don't humm at 60Hz, but what now looks like a square wave and that abrupt chop excites them like hitting a tuning fork. Sometimes, rough-service bulbs (like those designed for a garage door opener) have the filaments better supported, and they don't humm. It's trial and error (which can get expensive!) to find a pair of bulbs and dimmer that minimize the problem.