@mwinkc it's a (not very safe) bypass on ground, so you can rule out ground loop issues. I think you're right, it's environmental. Unfortunately I don't think it's related to electricity, it's probably radio interference. Because the only thing that made the sound almost go away was wrapping the thing in a faraday blanket.
The disappointing part is that I can't fix that-- it's probably coming from outside. So I'm wondering if SS will be as sensitive to RFI, or if I can get away with hybrid.