You can do your own line test by the following:-
Unplug any extension phones, extension cables, answer machines or fax (anything except the phone you will use to do the test!).
Plug a normal touch tone phone directly into the BT master socket.
Dial 17070, press option 2 (quiet line test)
You should hear 'Quiet Line Test' and then silence, there should be no pops, clicks, whistles, buzzing etc. If there is noise on the line, make sure it's not your phone's connection to the socket (wiggle it about a bit) and that you are using the master socket. If you are sure its the line making the noise then dial BT and report the fault, they should be able to sort it out. Remember that 'mis-reporting' a fault (e.g. if it turns out to be your phone, extension cord etc.) may be charged a call-out fee by BT.
Hope that helps.
Oh, by the way, you have got a filter plugged in if you're on broadband, haven't you?