My line also has ongoing intermittent audible noise issues but I can't escalate it without threat of a fine from BT so long as their line tests reveal no problems.
What is it with such ignorant people who don't believe they should pay where they have created a problem?
Get an Openreach engineer called out, and the problem is provider or Openreach kit that isn't picked up in remote testing, no charge. If it is because you have bought a cheap set of fairy lights that are spewing out interference then you pay.
It is exactly the same as having a maintenance contract for your central heating. Thermostat in the boiler fails through age, engineer comes and fixes it under contract. You try bleeding a radiator and break the thread on the bleed valve and there will be at least a call out charge. You don't hear people bleating about British Gas threatening to fine you for their problem.
Or would you prefer that you aren't advised that they will only fix their own kit and then send you a surprise bill you when it is your fault?