I agree with Paul.
What type of noise is being observed?;
Is it a "Predictable" pop?
Is it a string of pops and hisses?
Is it a steady hum?
Is it like a Dentist's drill?
Is it all of the above?
Predictable pops would be from the toaster's switch opening and closing.
String of pops and hisses would be thermostatically connected elements going on and off, and maybe sounds coupled from other sources.
Steady hum would be some type of reactive elements' coupling.
Dentist's drill would be Solid State control elements (or other stuff coupled to it).
All above could be anything!!!