For a while (maybe a week or two - can't really remember when it
started) now we've been trying to work out what has been causing this
odd pop/bang noise from outside our house. After a bit of elimination,
and finally this morning, sitting looking out the window for ages, we've
determined that it's our neighbours boiler.

What appears to be happening is that when the neighbour's boiler (fanned
flue) starts up, immediately before the normal plume appears there's a
pop/bang and then the plume starts.

The neighbours have been away for a few days, but I think it may have
been happening a bit longer than that - but until today were unable to
say with any certainty where it was coming from as it appeared to be so
irregular. The only thing I could say during this weekend was that,
during daylight, the boiler was always pluming when we checked after
hearing the noise - but obviously, until we were watching when it
occurred we didn't know whether it came from somewhere else entirely.
We didn't want to start apportioning blame without evidence (had that
with a different neighbour claiming we interfered with her TV reception).

We'll be (if my wife hasn't already) notifying our neighbours about it -
but I wondered what the cause of the problem might be, so our neighbours
can at least know whether anyone who looks at it is giving them honest
advice.

Thanks

David