A lot of faults with noise is caused by internal wiring
I know, that was my first port of call. Done all the suggested things, also tested with the test socket for days on end.
I even know the quiet line test number.
Does the crackling occur when its damp / raining outside?
Er.. yes, but that’s only because it’s been damp / raining forever 🙂 Actually, it started a few months ago, which may have coincided with the dry weather thinking about it.
The crackling is really loud, like bad interference, and it goes on for a while, then it’ll suddenly stop and the line will be clear as a bell, for another while, then it comes back.