[RFI] Power Line RFI... Solved (I think)

[Andy]

That's good that you found something concrete.

It is possible that it was the switch contacts.

I think there are many other possibilities too.  For example, since
you interrupted power momentarily, many electronic circuits in your
house will have noticed the momentary drop-out and may be behaving
differently.  A charging circuit might have flipped from "fully
charged" to "charging".

I'm also guessing that you may have had multiple RFI sources, since
you also noticed some of the noise dropped after a rain.  That hints
at something in the power lines (or perhaps an electric fence), not
your transfer switch.

Andy