[squarefoot]

As others replied, you may have grounding problems, that is, either the lack of grounding or too much of it (ground loop). An effective way to solve the problem is to isolate the signal by putting an audio transformer in between the outputs and the amplifier (or amplified speakers), one for each output. I've done this for desktops and laptops and it brought the noises to zero. Just make sure the transformers are of decent quality and suitable for audio.