Which of the following can create intermittent loud roaring or buzzing AC line interference?
Why Power line type noise is really arcing: any spark gap repeating at the AC line rate (60 Hz, so buzz at 120 Hz and harmonics) radiates broadband RF that sounds like a loud roar or buzz. Thermostat contacts in furnaces, water heaters and refrigerators arc each time they make or break, so the noise comes and goes with the appliance cycle. A failing doorbell or its transformer buzzes continuously through a partial short or arcing contact, and neon or LED advertising signs with failing tubes, ballasts or loose connections arc the same way. All of these are intermittent because they depend on when the device switches on or how the arc wanders.
Watch out Picking just one source is the trap; the pool groups all the common arcing culprits into a single answer because they all produce the same symptom on the radio.
If it arcs, it buzzes. Thermostat, doorbell, neon sign: all spark gaps on the 60 Hz line.