Hello, Echolane and good evening to you sir. If
I were in your situation, I would K.I.S.S.
In other words, go the simple route first to diagnose the issue. With that said.....if the issue isn't audible within your system? Is there really an issue?
When you have the house to yourself, turn off every breaker in the home but the one(s) powering the audio system. Run the test again. Is the result the same? If so, you have isolated the culprit circuit and we can move forward. If not? Keep flipping one breaker at a time and retest
again until you do find the culprit circuit(s) that may be causing the noise.
After this is done and you have isolated the circuit(s), only then can we isolate the issue further IMO/IME.
Please let me know what you find out. Otherwise, this could be a 30-40 page thread without you finding out anything whilst everyone here is just shooting in the dark as to "possible" culprits. Thanks and have a great evening!