Thanks for the reply, musicman691. I didn't explicitly state this in the comments for the YouTube video (because I've tried everything), but the A7X units that have made this noise have done so:
- with or without a cable connected to one of the inputs
- with the front panel level control all the way down, all the way up and everywhere in between
- with the level on my monitor controller all the way down, all the way up and everywhere in between
- connected to the sub's satellite out and not
- in my studio (with separate circuit for gear and lighting)
- in my house
So bottom line, it's not any kind of interference from an input signal. Completely independent from input or output level. It's a noise the speakers are making themselves.
Brian