Hissy noises slightly modulated by 120Hz or multiples generally come from
real high voltage power lines (Three phases can make 360Hz buzz). You get
two noise peaks per AC cycle.
That's because the arc fires early and last for a long period, so the ratio
of on-to-off time is very large. More time for the arc to sizzle instead of
just popping on and off quickly at a peak.
LV lines are sharp and raspy. HV lines often (but not always) are hissy and
hard to blank.
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