Is DC Pedestal fan motor whine normal at low speed?
Dead common, but it's more often the circuitry providing the DC voltage to the fan, in my experience.
Not clear what you are powering this from (AC/DC/Voltage...), but the speed control probably consists of some sort of switching circuit, and when there are electronic components that are being switched at audible frequencies, you sometimes get noises you can hear - often quite irritating.