I think what you actually hear is coil noise from the fast switching of the current in motor's coils. The motor they use looks and sounds more like a HDD motor

I think what you actually hear is coil noise from the fast switching of the current in motor's coils. The motor they use looks and sounds more like a HDD motor more than a normal fan motor - optimized for constant speed under quite a heavy load. The fan motors work in a similar way, but they use less power and less coils in a bigger enclosed space, so they can be better optimized for sound.