i too am getting humming. i'm pretty sure it's coming from the fan. i'm just using a 600w light dimmer switch. my active air 6" inline fan, my 20" box fan, and my 12" oscillating fan all do this when plugged into the dimmer. buzzing gets louder as you turn the dimmer up, then almost drops off when turned all the way (still buzzes a little). the frequency of the sound is the same at all dimmer settings, it's just the volume that changes.
when i plug the fans into an outlet directly bypassing the dimmer, there is no humming/buzzing in that initial time where the motor is powered up but the fan hasn't gained enough momentum to reach full operating speed. with the dimmer, there is a humming/buzzing sound during that initial spin-up period.
the box fan and oscillating fan have a low-med-high switch which produces no buzzing when dialed down, so obviously it must be using a different mechanism than the dimmer.
this guy he says you'll severely shorten the life of the fan using a dimmer. basically, it will burn out the motor.
http://www.420magazine.com/forums/how-grow-marijuana/72074-can-i-use-dimmer-switch-control-speed-my-fan.html he says you can use something called a variable frequency controller to slow down a fan by changing the frequency of the alternating current. they seem pricey and more for industrial use ($100+)
so i'm off to the hardware store to purchase a three-speed fan controller ($20-30). these have a low-med-high setting similar to those found on most household fans. i have a feeling it has three speeds because it has 3 things (whatever they are) that are preset to certain frequencies (i'm hoping). that's why it's not just an analog dial that you can set to anything between 0 and 100% (like a dimmer). i'm hoping fan speed controller is, indeed, a variable frequency controller. after all, it is designed to control the speed of a fan!