It has already been mentioned but my response to this was to seek out a ground loop isolator. This is the best way to get rid of this noise which plagues every laptop that you plug into an audio rig of some sort. They work fine when powered by battery, but the second you connect them to a power suppl....nooooise.
The other method to getting round this, however a little more risky, is to remove the earth from the plug. It's the earth that causes this noise.
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