Hi
All you can do is monitor the time to see exactly when the interference starts. If it is exactly the same time each day then usually points to some machinery or equipment being turned on automatically. It's easy to rule out things in your own property, but next to impossible to track something down locally, and even harder if you do find the source of interference to get anything done about it.
If the time varies somewhat, my guess would be this is someone turning on and off their VDSL modem, i.e. they turn it on in the evenings when they are home, and turn it off later when they go to bed to be "green" to save electricity, or they are the sort of person who worries about electrical fires and turns things off. This means you are just seeing cross-talk come and go and a reason why BT prefer VDSL modems to remain on 24/7 as these sort of fluctuations can get reported as faults.
Regards
Phil