Probably a silly question, but how have you eliminated Neighbour #1 and Neighbour #4 from being the cause?
I haven't directly eliminated them, no, because the apparent centre of the noise was either myself or neighbour "3".
Neighbour "3" doesn't have an alarm, and aside from a programmable thermostat for their heating which seemed to be battery powered, I can't see anything else that might be making noise without mains power. The stat made no noise, with or without power!
The noise was a lot quieter around "1" and "4".
HOWEVER
I've just been round to neighbour "3" for a chat, and took my radio to demonstrate that the noise was gone today.
Or, as it turns out, gone in my house,
but still present in theirs, albeit quieter than last night.
And the strongest source is their party wall with "4". If I go into "3"'s garden, hold the radio close to the house and the party wall, it's worst in that corner (back corner, farthest from road).
So it looks like it could be coming from "4".
A walk around the block shows that I can't detect it at all from outside, only in "3"'s house where it adjoins "4"
But it's odd that last night the strongest point appeared to be centred between "2" and "3". Now it's between "3" and "4".
How could this be?
I don't know the people at "4" at all, so I've go to play this carefully so I don't look mad, turning up on their doorstep asking if I can walk around their house listening to noise on my radio....
Ian