FAN NOISE issue in triac phase angle dimming
I have also experienced the same humming noise while using PWM dimming on Fan. The only solution that works well is by using capacitors to reduce speed. You may need to stack 2 or 3 capacitors (1uF, 2.2uF, 3uF) which are rated to work in 250VAC and connect them in series to get various speed levels. It is a little complex and bulky compared to PWM approach, however this will not create any humming noise.