I have a 24" iMac, today picked up a of Bose Companion 2 speakers to replace my old Boston Acoustic speakers which I thought were faulty with the same buzzing sounds. The problem is that after the computer sits idle with no sound for about 30 seconds the speakers begin to make a humm/buzz noise. I can move the wires and nothing changes. I've spread the speakers as far as I can, and it continues. I moved the Bose adapter to an extension cord and ran it about 25' to an outlet on another circuit from the iMac, still humming. I've timed the time it takes from no humm to humm and its 30 seconds. My gut is telling me that the iMac is shutting down an internal amp to save power or something. Is there something I can change to eliminate this, or is this a fault that needs Apple's attention?