AC Dimming of FAN making humming noise
There are a couple of causes of this noise.  

1) The sharp rising edge of the incoming power causes mechanical noise.  About the only way to reduce or eliminate the noise is to use a large inductor in series with the phase control circuit to reduce the rise time.

This was and still can be a significant problem with Theatrical lighting dimmers.  Their fix is to include this physically large and heavy inductor within the dimmer chassis - one inductor for each lighting channel.

2) Your dimmer is not conducting equally on both the negative and positive half-cycles.  

Many inexpensive dimmer circuits do not work well with inductive loads.  The problem manifests itself by missing half-cycles of the out-going AC waveform.

This leads to significant noise and a lot of heat.  Because you are then feeding essentially half-wave rectified DC to the fan, it makes a lot of noise and will soon fail.