Good Evening,
The wall switch uses a Triac to replace a contact. This is a solid state device that can be gated on when the AC is passing through the positive or negative half of the cycle. It can be gated on at any point but will not turn off until the AC goes through the zero current point (capacitance and inductance change this from the zero voltage point). The triac then must be gated to turn on each ½ AC cycle. The Triac is not truly 100% on so you are getting a chopped sine wave into the motor causing it to vibrate mechanically causing hum or the noise you hear. The X10 switch is ok for the light portion and will work great but the Motor does not like the modified sine wave and complains.
The X10 switch will cause the motor to operate hotter possibly to the point you can destroy the motor and or X10 switch. It will slow the motor if it does not destroy itself. This is caused by the increased slip as the gating of the triac is cut back i.e. Going from 180 degrees per half cycle to zero. Loading of the motor etc will have a large effect on the slip and consequently the speed.
Triacs can and are used to control motors without problems but the circuitry using them is different then when dimming lamps. The inductive component of the motor complicates the control circuitry increasing its cost. Also when doing this with a motor a universal motor is normally used not the typical motors (probably variable reluctance) used in ceiling fans.
A simple rule around the home is motors and florescent lamps use contacts tungsten lamps use triacs or contacts. This has many exceptions but they have to be examined on a case by case basis.
Hope this helps
Gil Shultz
