Noise due to long phone line

I wonder if any of you can help me with a problem I am having with my ADSL modem.
At the moment my modem works fine but the problem is it is making a high pitched noise.
I have pin pointed the problem by unplugging the wires one at a time.
It seems when I plug in the phone line into the modem, the modem starts making the noise. I should tell you that my phone line is 15 metres long.
15 metres from the modem to the phone socket. It can not be any shorter as the computer is in my room . I have called the electrician and he said it will be almost impossible to install another phone line in my house as the setup of my house is no good for another phone socket.
It is a D link 502 T
Is there an attachment I can buy or any thing I can do to get rid of this noise?