Which of the following can create intermittent loud roaring or buzzing AC line interference?

Intermittent loud roaring or buzzing AC line noise is almost always the result of electrical arcing. When contacts in switches, relays, or thermostats open and close, or when components degrade, electrical arcs can form. This arcing acts like a broadband spark-gap transmitter, emitting intense RF noise. Thermostatically controlled devices (like heaters or electric blankets) have mechanical contacts that wear out and arc over time. Defective doorbell transformers or malfunctioning illuminated signs (like neon lights) also commonly break down and generate sustained arcing. Since all of these devices are notorious for creating this exact type of broadband interference, they are all correct examples of potential noise sources.