I have a feeling that this is going to be a pain to troubleshoot as it's intermittent. Running an online line test kills the internet connection, so I never see the result, but weirdly seems to fix it for a bit.
Any ideas on how to approach this with BT, and not get a useless engineer visit that we have to pay for?
Here's an audio file of the crackling (sorry for the traffic noise in the background): http://www.filedropper.com/linenoise
Nothing changed internally etc.