Is it from the electrical (?) cabinet near the noticeboard on the corner by any chance?
I think modem and other power supplies in the home can turn out to be the culprits too. Might be worth walking around turning things on and off to see if any particular thing causes changes.
If there is a large source of electrical interference in the area generally it might explain why Three 3G is problematic/has drop outs, and why the noise margins on so many ADSL lines are at 15 round here and perform poorly.
So what's required to 'build up a history', do you need to report this every week and have an engineer out every week finding the same time and time again before someone then investigates it?