I would suggest if it is a power filtering issue that you could try using a choke on the aux power connector. Something similar to this
, a $5 radio shack part. In general it sounds to me like it's induced by ripple in the supply. Which could be from crappy PSU filtering, poor line power, poor chassis ground on the PC/monitor, poorly isolated analog and digital gnd planes on the video/motherboard PCBs, poorly earthed wall outlet, recieved EMI, etc., etc... not exactly something that is easy to isolate from a consumer standpoint.