I have a brand new DC motor pedestal fan with 12 speeds. At the very low speeds the motor makes a whiny bearing type noise. Is this normal for low voltage DC fans?
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.
DC fans can be brushless or direct drive (Servo). The sounds could be caused by the low DC voltage generating oscillations in the stator windings. I don't have a whole lot of experience with DC motors. However, I see this issue a lot in AC replacement motors that are poorly manufactured overseas. If it's a new fan, I would return it for another one. It will probably only get worse.