Impiulse noise - that is in general outside BTs control. If the ambient noise level increases and it can be hundreds of items that cause it then teh line performance will change. In a most cases the change is gradual and slight so the effect on te line is small.
However, if there is a large change in noise - a large burst for example then the router will see this and compensate/readjust. An example could be a back up generator, or a noisy electric motor being used close to a BT street cabinet or above a cable duct.
Two examples I have seen: A friend of a friend asked if I could have a look at their broadband as it was dropping out from early evening through to just after midnight, some nighgts. Their neighbours were having problems too. He left me there when he went to work - about 1/2 mile away and said, when you are hungry come down for a kebab (he had a mobile takeaway stand). After about 30 minutes the circuit dropped and restablished at a very slow speed. I walked down teh road and there he was, in the layby with his generator right next to the cabinet. The reason it was only some evenings was that his location in the layby varied.
Secondly, when repairing a road and footpath the workment put their compressor right against pole - killed all broadband to the 8 houses served from that pole.
Are either of those BTs fault? No.
