Analog is not my thing - and definitely not audio, but how bout this. Since it doesn't make sense to be getting noise from the fan on the wires, maybe it is radiated from the motor. After all even in brushless there is still magnets spinning and such. Try moving the fan around and see if the noise changes. Now if it does change, I will still be of no value to get rid of it, but at least you will know the source.