You may need to test your system with the less components installed possible, for example just test the motherboard, with power supply, video card, processor, fan and Heat Sink and one stick of memory. Remove any other components installed. If possible test the motherboard out of the chassis, placed on a non-conductive surface like a newspaper, a magazine or something similar. This would be to isolate this from being a noise from a chassis fan or a noise coming from another component. In addition check if the noise comes from the processor's fan or the video card's fan (if any is present).