Run up DMT using a DG834, preferably a V4 and you'll most likely see a large notch in the spectrum due to (probably local) RFI... Tune a scanner or MW radio to the frequency indicated to hear it and simply follow the noise souce to the troublesome device.
Possible causes (I have found this way) include:
PC PSU in standby mode
Low energy lamps
Old CRT Monitor
Netgear ProSafe Switch (NOT the PSU)
Various plug-top PSUs
Unless of course there's an RFI Filter in the ADSL path... That'll do the same...
Edited by TechGuyUK (Sun 17-Feb-13 17:14:51)