maybe i can help you out i just got rid of this problem on mine ok you said the pc is using the aux in port on the headunit if you unplug the audio cable fromt eh soundcard does the whine stop?
If you turn the PC off is it still there with the headunit one? at a high volume?
and next was my problem I used a plexiglass sheet to mount my motherboard well needless to say the brackets on the plexiglass were some how causeing a whine I insulated all the connections and bolted everything back down and it stopped.
oh and I am running mine off of a old 400w generic inverter