My guess would a device that is doing some sort of communication check. Someone earlier in the thread referenced a computer being too close-----could be that device pinging the internet (notice your modem lights are constantly going), or even interference caused by the hard drive spinning. Ever been on a conference call and you can hear an intermittent buzz when someone's mobile phone is just checking the airwaves (I know, not the right term)? Something of this regularity makes me think communication check vs something cycling (refrigerator, A/C, etc.).
If you are using surge protectors, try putting the Amps on one by themselves if everything is already on one surge protector. That is really just a shot in the dark because as someone else also pointed out, your issue doesn't sound like a ground-loop hum. Is there such a thing as an intermittent ground-loop hum? If you can isolate stuff like modems, routers, switches, and your computer, maybe that will help. I would think isolating the amps would fix this if that is the case.
