I would try a different circuit breaker when you change the plug for the TT, not just another outlet. Is it possible to turn the plug in the wall socket, sometimes changing the polarity will help (I have older TT's, always the first thing I try)? If the plug is "keyed", one wide one narrow blade you will not be able to do so. At the hardware store you can buy an inexpensive outlet tester (everyone should have one), it will look like an extension cord end. It will have LEDs on the back and the combination of the lights will tell if the wiring is correct, polarity, ground, etc. You probably are hearing 60 cycle hum, which is the frequency of A-C power. One outlet or lamp wired incorrectly, motor loads, nearby un-shielded transformers, bad ground or voltage on the neutral.....BTW some MM cartridges are "sensitive" do you have another to try? good luck

My house was built in 1905, and two owners ago, had an Electrician in residence. As a result I am up to code, at least NEC from 20 years ago  Consult with your electrician,,, A single breaker panel with a clean neutral and an isolated ground will do wonders. You will then have an orange, isolated ground, outlet, dedicated to a single use. Especially in older houses circuits are expanded, lighting added, extra outlets, and so on. All too often the ground is omitted altogether, and the neutral is picked up and used as a "common" wire and has too many connections on a leg which can lead to potential (Voltage) on the neutral, which is my SWAG for your issue, You might want to have an electrician look over your service, panels, wires and outlets/switches. Hope you're sitting down when you get the report. Having things brought up to code, though, will give you peace of mind (safety), improve the performance of your system and computers, AND increase the value of your home.

Keep us updated, and good luck.