R, ofttimes, cheap cabling can be a culprit. I think you're on the right trail, Tonto, now it's perseverance you'll need. Some noise problems would peeve Job (not Steven).
Best to you on finding a solution.
I'm sorely tempted to hire Guido, if only for the beer und shashlik. lol
Personally, I suspect a earth (ground) loop.
Maybe I've missed it, but do we know the hum frequency? That would help isolate PSU or mains. Does it sound like your Barber's clippers (mains) or a harmonic thereof (PSU)?