I don't want to overly complicate matters but there are two types of interference that could be the problem. I've simplified a little (hopefully?) :
a) radiated - something is causing RF (radio frequency) transmissions to take place where they shouldn't. The radio trick kitz mentions might be useful here. I'm not 100% about the frequency she's chosen but I'll defer to her here as I've had little to do with ADSL (rather than ADSL2+) in the last year or so. It may be of limited use anyway - eg the equipment in my house which causes me minor problems radiates interference at about 2.1Mhz (2100kHz) and is undetectable 3 feet away. It's not only radiated interference that it causes though;
b) conducted - this is where the interference is conducted along wiring (your 240V mains wiring usually). The equipment causing the interference is introducing the interference from the power supply (which will almost invariably be switched mode these days) within the equipment. This doesn't necessarily mean that the equipment is faulty but rather that the designer didn't pay too much attention to minimising interference. The usual suspects are what I refer to as "wall warts" - you'll all have some, they are the hideous big black (usually) power supplies that are in the shape of a plug. They are usually cheap as chips and almost all of them induce noise into the mains. In my case the equipment that radiates also has conducted interference which is the part which causes the issues - I suspect if the location of the master socket was elsewhere I'd be OK but the hassle of moving it outweighs the gain
Feel free to ignore the next bit if it flies over your heads:
You can also have a combination of both. eg I put in CAT6e cabling a few months ago. Each pair is individually screened, then all four pairs have an overall screen and there is a centre conductor in the cable. I didn't bother earthing the centre conductor (bad bad rizla) as this was my first experience of terminating/crimping CAT6e and all seemed well. Then I noticed interference on a CCTV camera* - wavy lines. Then I noticed that the ADSL had dropped/resync'd at 16Mbps. It was in fact the equipment mentioned in a) radiating interference which was picked up by the centre conductor and then re-radiated to the telephone line (and CCTV cabling) at one point around the back of the house. Earthing the conductor stopped that dead. I'm comfortable doing all this as I used to design radar systems so I can track it down (usually).
In short interference is a bloody nightmare for the average ADSL user.
*we have a couple of CCTV cameras - they use less power than security lighting and are more of a deterrent to the scroats off the local council estate.
