When installed to regulate ceiling fan speed, the fan hums at the low speed. A dimmer switch which I had previously installed on another fan does not. The other dimmer also has 3 distinct positions, rather than continuous adjustment. I don't care about that, but I do about the humming. Did I buy the wrong switch, and if so, what model should I have purchased.The switch is in a box with other switches, so must look like the model I purchased.