A walk around with a portable shortwave radio seems to indicate it gets stronger the further towards the back of the block I get.
It's not sunny today so I doubt it is any type of solar inverter. To me it sounds like something that is sparking, perhaps a motor?
Here's a short video where you can hear the sound it makes.
This is going to be a real challenge to overcome.
This morning I noticed that the interference had changed to broadband noise but then it switched back to the pulse style. I went for a walk along the lane behind our house.
It seems to be maximum strength behind the neighbour's back shed. I'm wondering if it's a solar charge controller or similar.
The noise has stopped. The noise floor is still pretty bad but the terrible pulsing interference stopped some time on Saturday afternoon.
As we say in programming: bugs that go away by themselves come back by themselves. I will continue to monitor the situation and improve my antenna.