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?
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.