Yes, but he also has interference with no horizontal directionality. And there's much less of it when his guitar faces up or down. That suggests that the source is all around him. What's all around you but not so much above you or below you? The wiring in your walls. It exists whether the breaker is on or off.
Long shot? Maybe. But I've never seen this set of symptoms before.
2 things-
You said 'faulty ground.' If the electricity is off, any ground condition, good or bad, now effectively doesn't exist, and 2,
House wiring doesn't just go in the walls. In fact,
most of it gets stapled up under the floor joists, and above/within the ceiling joists or trusses. The main feeds will typically be across the ceilings or under the floor, then each circuit will drop down or up,
then run through walls. I've seen many houses where the electricians wire the walls by going from each outlet, either up the stud, across a few joists, then down a stud to the next box..., or, much more commonly, run completely under the floor, and only come up the 16" at each outlet location, and back down again. Which in that case, practically all the wiring would be in the floor. So it's really both, and quite dependent on how the electricians decide how many holes they want to drill, or not. It's only at the ends of the "home runs" that then go into a wall.
OP, you wanna rule out house wiring, go stand near your breaker panel, and walk around in that area, and away from it towards other parts of the house, both with the power on and off, and report back. You could also turn all the circuits off except for, one at a time-
Dryer
Washer
Stove
Water Heater
then walk from your panel towards those appliances and see what happens. I'd bet there's no changes to the interference, since I think the source is external.
