All the times I've heard people refer to a humming, static of buzzing sound, especially when they've ruled everything out, it winds up that they either had dirty power, or they were running a Back-UPS unit that wasn't compatible with Active PFC power suplies. With PFC power supplies, like the iMac has, you absolutely must run a Pure Sine Wave Back-UPS unit. Or else you'll get that humming. Also, most of the units, like the one I use, the CyberPower CP1350PFCLCD, have an AVR feature, that conditions, and regulates the power voltage without using the battery, which takes care of brownouts and such. You'd be surprised how much stuff can affect your home's power. In my house, if I turn on my laser printer or space heater in the same room, on my Back-UPS unit, I notice the input voltage drop quite a bit. Sometimes to the point where the Back-UPS unit would kick in for a second.