I have a stereo set-up with a sub-woofer. Sometimes, if my laptop is plugged in close by or in the same outlet, the sub-woofer "hums". It's not like a rumble, it's like a long, low, note. An engineering major told me that it had something to do with magnetic fields being generated or something, but I wasn't entirely sober when he was explaining it to me. What could be causing it?