Following on from what Kitz said, if you are lucky, sometimes timing can be a clue ; if interference changes at certain times of day then that helps with the guesswork as to what the source might be. Central heating and street lights go on and off and do so at certain fixed times, although central heating might instead go / off according to a thermostat as well as being controlled by a timer of course, and a user might press a button to control it, eg "+1 hour". I have a possible weird source of interference which, if the understanding is correct, goes on and off in a pretty regular pattern every day, but I haven’t been able to make sense of the meaning behind the times, it only affects one of my four dsl lines, and only the upstream, while two other lines are completely unaffected and one is much less affected but not with the same timing pattern; rather with no regular pattern at all.