If you have access to an oscilloscope, you can put it on the mail lines and see if anything is riding on them that would be causing this. You mentioned the solar panels as possibly having a carrier. Sweep the freq up and down and you'll see what's hiding there.

I'd certainly do the laptop powering the interface and see if it's still there. If it is.... I'd then carry that setup a good distance away to the local pub or something that gets you away from any local noise sources.

This certainly sounds like a detective job for you.

Since you are in AUS.... any aboriginal graveyards nearby?

Do keep us posted when you finally figure it out.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.