Do you have another AV unit that you could use to test to see if it is AV Switcher. I know that you said that other componants work the AV Box but it is worth a try.
Try grounding things in a different location.
Try unplugging all inputs from the AV box and take the RCA from the computer and touch the chassis of the AV Box. If you hear the noise then it is comming from your computer.
Check the ground on your computer. Is the computer in a metal case. Ground the computer to your PS if not. The best ground location on the via is the screw hole next to the PS/2 connectors.