What about harmonics from wireless routers? Could that explain this
interference pattern? - Doug

It seems very unlikely to me. That stuff is up in the GHz.
They don't tend to make subharmonics.

The carrier frequency is around 2.4 GHz, sure, but the data bursts are quite
a bit slower. What I don't know about is whether wireless routers broadcast
data bursts at regular intervals that somehow may be getting demodulated in
my receiver.

My computer monitor has a 62 kHz horizontal sweep.
There are various places on HF that I pick up "carriers"
evenly spaced at 62 kHz. (Antenna is outside, connected
with coax.) When I switch off the monitor the "carriers"
disappear.

I use an LCD panel. Turning that off makes no difference, as expected.

One thing to try is to remove power from everything
in the house. You can't just turn them off, you have to
disconnect the power. I have a Sony stereo that makes
RF noise if it is just plugged in.
You can switch off all the circuit breakers to everything
but the radio. (And unplug anything left on the live circuit.)
Or if you can power the radio with batteries
then switch off all or the main breaker.
See how much noise goes away then.

I've gone through this process when chasing down a nasty buzz in the AM and
SW broadcast bands. I found that the light dimmers on my outdoor spotlights
caused a nasty buzz all across the bands, not at any kind of regularly
spaced frequencies. That sound was also quite different than what I am
getting now. I'll have to explore that idea some more.

Thanks, and keep the ideas coming...hopefully we'll figure it out.

- Doug