Guitar processor noise...? BZZZZZZ!

If he put a stereo jack in there, the ring contact may be interfering, and you must make sure that the shield and tip actually go where "sposed" to. Sound like you are on the right track

Years ago, when installing the E911 stuff, this is when CRT monitors were getting BIG. And of course the fed-assisted 911 projects could afford BIG. And the BIGGER the CRT, the more RF noise they make.

We had installed a Motorola CRT "Centracom" radio control console, which has a balanced cable comes out, wherever you want to mount it under the desk, etc, for the operator's headset/ mic. That cable, balanced, was deemed "not needed" to be shielded by Motorola. Well, "news." It needed to be..........I actually found an old piece of coax in the sheriff's "junk box" in the basement, stripped it and pulled off a length of braid, and bunched it up "Chinese finger" style, and worked it over the top of the cable. Then grounded the end next to the console. I was a "hero."