I have a Cambridge Soundworks subwoofer attached to my mac pro and has just been emitting intermittent static noise regardless of whether audio is being played through the system. The static only seems to affect the subwoofer and not the satellite speakers and continues even if the input volume is turned down on the unit and in OS 10.5.7. I have moved the power adapter to different outlets and away from other power cables but that has not made a difference. I have an APC battery backup unit about a foot away from the subwoofer, but it has been in a similar position for years without the static problem that recently began. Any suggestions?