Guitar processor noise...? BZZZZZZ!

AC interference can be truely weird. Back in the 90's I had a radio amateur friend who had an Astron variable DC power supply on his desk. I honestly don't remember "which way" this went. Either the power supply would screw with a dimmable desk lamp, or the other way around. We had a similar situation at the Motorola shop for a short time. Something about a soldering station and a power supply or lamp that would interact. "No good logical reason." One of them was generating some sort of "hash" on the line that was "getting into" the other component.