Fact or fiction?

[#1]

I am struggling to understand a problem with a SNR (and hence the connection sync. rate) on the upstream signal which is very variable.  Does this imply that the source of the noise/interference is local?  Or is it just indicative of a problem at low frequencies that could be anywhere on the line?
The former hypothesis was put to me by the Demon tech. support rep. I was talking to about the problem (this issue is not a Plusnet one but I know this forum is a good source of expertise).     

[#2]

It is a problem at low frequencies somewhere on the line
I had exactly that with uncapped upstream on my line so went back to capped
I also get occasional noise on the phone line and things can go badly wrong in very heavy rain - masses of uncorrected downstream errors - several BT visits didn't sort it and as the feed to me house comes from a shared pole (with electrical supply to another property) getting it investigated properly is impossible so I live with it