For some time I have noticed a sawtooth pattern on my SNR/Tone graph. At first I thought it might be due to interference from a DECT phone, but unplugging the phone and connecting the modem/router to the master socket eliminated that as a source.
Then I compared the results with others on MDWS (which is now in its final few days), and it seems to me that everyone reporting from the UK that is on Vectored VDSL shows a similar pattern above a SNR of 40-45dB.
It would appear that Vectoring introduces the phenomenon. I have no idea why or how it happens, and I only mention it now out of interest.