You may have a ground loop, something thats pretty hard to isolate, i purchased an Asus notebook a little while back and had to return due to a real loud noise when launching the Mbox. You could look at some sort of grounding device. I took a look at the Behringer site the other night and they have an inline device which balances the signal, it may be the problem.
Cheers
Berhringer Hum Destroyer
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