Grumbling Sub

As soon as I hooked my mythbox up to the TV, my subwoofer started rumbling along with the disk seeks. This was actually quite amusing for like the first 2 minutes ;-) It continued even with my TV off. In fact, as long as my AV amp and sub were on, it would rumble. My TV and sub are not connected directly but they do share ground via the amp.

I was thinking I'd have to start changing power sockets and isolating ground connections when I remembered that my sub had ground switching. To my (and my neighbours) relief, flipping the ground switch eliminated the problem entirely! One down, 20 to go...