[VHFcontesting] Help Identifing Noise Source

[Mike Fahmie]

I've been suffering from a strong (S9+10dB) noise source on 6M for several months.  It tends to be present from about 10am to 4 or 5 PM.  It is absent most weekends but not all weekends.  In one case, it lasted all night!
Viewed on a scope, it comes in bursts at a 120Hz rate, each burst contains a sporadically random number of ~2KHz pulses.
It does not come on suddenly, but begins sporadically with a burst or two and gradually increases over an hour or so.  It goes off the same way.  Rain/overcast/sun does not seem to change its pattern.
There is a 12KV line across the street and the strongest noise comes from the direction of a vacant lot between 2 power poles directly across the street.  Beyond that is forest for a half mile.  One of the power poles has a transformer and a set of lightning arrestors (on the 12KV).  There are seldomly occupied homes on either side of the vacant lot.

I've eliminated my house as the source by switching the main breaker off and running the station on battery.
I suspected the lightning arrestors on the 12KV, but why would they only make noise during the day?
-Mike-WA6ZTY