Faint buzz in speakers

@OP Some amplifiers have small amounts of residual noise thought it should usually manifest itself as white noise, but there can be other noise as well. It is also possible that one or more of your noise reduction devices are causing the noise - including your balanced power unit. Take them all out of circuit and introduce them one by one. If the noise is still there after that, its most likely the amp itself. However, if you can't hear it from the listening position I wouldn't sweat about it.

Check the bug zapper in the garage 

Before tackling the audio interconnects, disconnect any video or ethernet devices. An RCA out to a TV that has a satellite cable in or an HDMI from another device plugged in elsewhere can be the problem. The additional shielding on “better” ethernet cables, like CAT-7 instead of CAT-6, can also be the problem. Ethernet is itself by design fine. But the shielding on a cable can be the problem...

Re: bug zapper comment above... yes, keep it off the house ethernet ;-)

Thanks to all who’ve chimed in about the buzz in both speakers. Perhaps it does sound more like white noise than a buzz. No, it is not audible from my listening position 8 ft away. In fact, it is so faint you have to place your ear right up against the cone to hear it. I don’t know which is more annoying, the noise itself or the fact that I’ve been unable to track it down. I’ll be sure to  try your suggestions this weekend when I have the time.