Speed Controller Fan Humming Noise

[Gazza1]

I have this speed controller and it's hooked up to a box fan that blows air out of one window while fresh air is being sucked in through the other.  The thing is if I use the speed controller instead of the 3 presets on the fan it makes a loud humming sound. 

How do you hookup/wireup a capacitor or something that will get rid of the sound?  I heard of people doing this.

http://www.amazon.com/SE-Electronic-Stepless-Speed-Controller/dp/B000HQAVNI