mystery signal

[nobody@xxxxxxxxxxxx (Dave Houston)]

I'm seeing a nearly continuous 2Vpp 120kHz signal on one 'phase'. The other
'phase' is clear. There are what resemble triac noise bursts approximately
1.6ms after each zero crossing and the 120kHz seems to be in sync with them.

I say nearly continuous because the signal is absent on what appear to be
random half cycles (again in sync with the triac-like bursts, 1.6ms past
ZC).

I say 'phase' because while the two circuits seem to have no communication,
they are in phase when observed on a scope. My apartment was originally two
apartments and there are two meters which might be a factor.

The signal is not coming from anything in my place but there are 4
apartments in the building and 4 more in another building across the street
which shares the transformer.

A random snapshot of 24 half cycles is 101101101110111110110110 with 1
indicating 120kHz presence and 0 its absence. I've been unable to discern
any clear pattern other than the OFFs never last beyond a single half cycle.

The triac noise pulses have been here for a few years but yesterday was the
first time I saw the nearly continuous signal. It blocks X-10. It shows up
as noise on my ESM1.

Any ideas?