Sudden change in SN Margin

[#1]

The SN Margin on the uplink  of my Thomson 585 v8 has overnight dropped from 13.5 to 7 after several weeks of stability  We had quite a lot of rain last night. Is this a typical outcome when the BT stuff gets damp? All circuits ar underground.
Edit: Forgot to mention it is standard copper ADSL

[#2]

Has you phone line by any chance become crackly? Upstream frequencies are at the lower end of the range next to those used for voice.
Could be dodgy joints in a flooded manhole.

[#3]

There's a few crackles on the quiet line test (17070 option 2) but not many.
The "normal" 13.6/6.5 SN ratio was restored when I restarted the Thomson Route but over the past few hours it's crept back to 8.5/6.0  I guess I'll wait until the sun shines.  The service is still running though a little bit more slowly, but I don'r want the joy of invoking an OpenReach djinn so I'll ignore it for a while.

[#4]

I may be wrong here but as far as I know the upstream noise margin reported by the router is set at resync and then doesn't change as it is actually measured by the exchange equipment
Your results therefore should be the result of a resync and not a creeping change
What I am sure about is that I have never had the upstream noise margin change dynamically as the downstream noise margin does