Probably a ground loop due to the radio and dell being grounded to different locations. Try this FAQ
I have a Dell Optiplex 260 hooked up in my truck with a rather simple configuration. I ran two wires from my battery back into my cab through the same firewall hole my radio antenna goes through. There is then a 15A fuse before power then goes to the computer. The computer works fine and everything. My problem is with the audio. I have a simple audio cable that goes from my computer to my in-dash radio. Anytime I have that cable plugged in I get a ringing through my speakers. It doesnt matter if I am on the radio, CD, or Auxiliary setting on my in-dash unit. The other weird thing is that this ringing increases in intensity as I start going faster and my truck's RPMs increase. Any suggestions of things to check? Im really at a loss.