The Christmas light controllers use SCR's and triacs
and those devices are an open switch until they are biased on at
which time they turn on very fast. That produces a spike of
radio energy so you get a pop at either 120 or 60 times per
second.

	I don't know how many of you have played with SCR's
before, but they make a good latching relay. On DC, they are off
until you bias the gate and then they are full on for as long as
current flows so on DC, they stay closed until you interrupt the
current through them. On AC, once you turn them on, they stay
that way until the half cycle ends and you are at 0 volts at
which point they open up again. That opening and closing is what
makes the buzz.

	A set of neighbors of ours across the street had lights,
one year, that cycled through many patterns and the sound was
loud and strange. It was a 120-hertz buzz with beats in it like
two instruments that haven't quite gotten in tune. After
Christmas, the air got quiet once again, at least from that.

	We've got a lot of corona noise where I live so when it
is dry, it sounds like somebody welding 24 hours a day right at
the end of the antenna.