Hi Peter,
Don't know if this is applicable to DSL, but several years ago I had a very
noisy phone line. I used Windows Hyperterminal (I think it was
hypertrm.exe) in accordance with the following article to find how much
noise was on the line. It's the only way I ever saw to measure line noise.
I recorded the readings, printed them, and explained them to a sympathetic
BellSouth repairman who went down to the corner and connected me to a new
"pair". The new pair was much better. It was a few years back and I don't
remember where I got this article, so I can't give credit. It was written
long before DSL. As I look at it now, you may need an old telephone modem
to use it, but that might be doable. I still use a telephone modem in
addition to my cable modem. I used to take the readings from the outdoor
phone interface (just run a temporary extension out the window) because
that eliminates inside wiring problems which are the homeowners
responsibility.
Here's a DSL FAQ    http://www.dslreports.com/faq/faq

Back in the sixties and seventies, before the phone company rewired our
neighborhood, the voice lines were often noisy. After every rainstorm the
linemen were at the neighborhood phone junction box. I think they were
changing pairs in response to customer complaints. I used to call it
musical pairs.
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Line Noise Overview
                 When To Suspect Line Noise
  With data calls, line noise can cause numerous problems. In general, the
faster you communicate, the better your phone lines need to be. When you
get up to 28.8 and 33.6 bps, it is critical that you have clear lines from
end to end. Line noise can cause the following problems when trying to make
high speed data connections:
                          No connection.
                          Connection, but only "garbage" characters appear
on screen.
                          Connection, but no communication.
                          Connection, then disconnection for no apparent
reason.
                          Connection, but modem speed drops.

              How To Check Line Noise
                 Connect to a remote computer or BBS using a terminal emulation
                 program such as Telix, ZTerm, or Hyper Terminal.

                 Type: +++ (OK, should appear on your screen)
                 Type: AT%Q (press the enter key) (you should see a number
on your
                 screen ranging from 000 and 255)
                 Type: A/ (a few times, repeatedly) (you should see numbers
each
                 time you press the enter key)
                 Type ATO (the letter O) to return from the command mode

                 Any numbers over 30, you have considerable line noise.
                 * Note: The above modem commands are not supported by all
                 modem manufacturers.

                 Where Does the Problem Originate?
  Line noise can be introduced at the remote site, by the phone lines, or
at your local site. Before making any changes, try calling a few different
remote modems, to see if you have the same line noise problems in each
instance.

One of the more common and familiar introduction points of line noise is in
the telephone company's system and even here there are several ways noise
is introduced. A signal is routed through multiple stations before it
eventually makes it to the other end and some of these stations aren't
exactly new. Older areas may have older, less sophisticated equipment that
is more apt to be affected by ambient noise. This is one reason some people
continue to have noise problems even after hanging up and calling back
multiple times. Your local telephone company is required by the FCC to
provide a minimum level of quality on the lines it maintains for your use.
Contact your local telephone company and ask them to perform a line
qualification test if you suspect this may be the problem.

Another common noise introduction point is in your home. Most residential
homes have televisions, radios, microwave ovens, VCR's, and if you are
reading this, a micro-computer. All these devices  radiate radio waves that
can (and often do) get into the phone lines and cause noise. Electric
motors and mechanical dimmer controls can introduce noise into the
electrical wiring in your house and cause problems. If your line noise
problem does not go away after repeated hanging up and calling back, then
you may be suffering from one of these household problems. If you are
suffering from this problem, you can take steps to eliminate it. First of
all, turn off EVERYTHING except the fridge and see if the noise persists.
If it goes away, then start turning things back on, checking the computer
each time until you see the noise start up again. It may be that a single
device is not bugging you but several devices plotting together to annoy
you. This elimination tournament may take awhile.
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>I have ADSL and tried toast.net for a speed check. It came in lower than
>what mcafee.com shows, but not a big difference. Toast showed 800kbps and
>mcafee read 1mbps. To me it is disturbing that my speed is going down - it
>had been at 1.5mbps when I first got it a few weeks ago! Still much better
>than the old dialup 33kbps.
>In reference to my speed drop, I suspect that phone line noise may be
>contributing to the problem. I get some static on the voice side (it happens
>after a rain). What else can cause such a slowdown in DSL?
>Peter