Line slowing down?

My ADSL2 line used to synch at 17Mb/s when I first had it upgraded. Since then I've had a couple of issues and had to have the SNRM fixed to 6db.

Now it's synching at just under 15MB/s with the same SNRM of 6db. Looking at the BT check it has recently changed to say that my downstream rate is "up to 8.5Mb/s" whereas it used to say "up to 11Mb/s".

Obviously BT aren't going to be in the slightest bit interested, but why is my line slowing down?

I don't hear any noise on a quiet line test, the line is running FAST etc.

I can only think that the noise has increased, albeit unnoticeably. What could be causing it?

Cheers,
AP