Ethervox background transient noise

I'm wondering if those of you who own -- or have worked on -- Ethervoxes have had any issues with "background transient noise" -- e.g., a kind of faint "musical buzzing sound." I don't know how else to describe it.

Mine has actually been doing this for a long time. It is isolated to the 1/4" phono output jack on the bottom of the cabinet and the "speaker" output jack on the snake box. It seems to be louder or more noticeable sometimes than others.

Voice1 and Voice2 outputs on the snake box do not have this problem, neither the 1/4" nor the XLR connections.

The recording engineer at my last Simpsons gig asked me about it. He said he thinks there's probably a shorted ground somewhere in the circuitry. I said I had no idea, nor any idea how to remedy it. I've just learned to live with it, that "it's a crazy instrument."

The sound engineer for the Bowl/Blue Man concert seemed to be concerned about it because he thought the sound would be audible in the house system. (Yes, they're going to mike me with a cordless mike. Yes, UGH. But as big as that place is, there's no other choice.)

Any ideas???