Right now, you're powering (and therefore probably grounding) your audio equipment and your source (in this case, carPC) in different places. Unless of course your headunit or amplifier is powered by the cigarette lighter, which is unlikely.
You have't told us what other audio equipment you're running, so I don't know how your system works. You didn't mention whether the noise happens both when the engine is on and off, so I'll assume that it's all nice when the motor is off.
Suffice it to say that the first, and biggest trick to eliminating noise is grounding everything at exactly the same place. This doesn't need to be at the battery - in fact the shortest ground connection is the best. By minimising the ground difference potential, you're helping stop those nasty noises.
Have a look at the sectioned headed Engine Noise on this page: http://www.bcae1.com/audiots.htm