NAD 3020a Interference

I recently bought a NAD 3020a second hand. It works and generally sounds brilliant, but there is high pitched crackly interference when it is plugged into my computer. There is none when I use my iPod with it so it must be the computer. The funny thing is, when my computer is off, the interference goes away. Does anyone know what the problem could be? I've tried ferrite beads and they didn't do the job.

Any help would be very much appreciated!