Guitar Amp Humming (interference?)
It’s most likely caused by induction- if the AC supply is more or less in a loop around the problem location then it will act like one turn on an air-cored transformer- very little energy will be involved but enough to create audible hum.  It’s can be rather difficult to pin down the cause (perhaps try pulling circuit breakers and running an extension lead to identify the circuit that’s the culprit, but resolving it is another matter.  Another possibility is that a badly-behaved load is causing interference on that circuit - try switching everything else off and see if you can isolate it.