Whenever I have headphones connected to my 20in Aluminum iMac, I can occasionally here faint but still audible blips and high pitch ringing, like circuitry noise. It seems to only happen when there is no audio being played and once audio starts being played the circuitry noise stops. Its very random. Does anyone know what could be causing this? There are no wireless devices (other than wifi..) and does the same thing with different headphones. I know its not an application because it does it when I have no apps or unnormal processes running. Could it be the GPU or CPU causing noise?