Okay, I have recordings!
The first one was recorded whilst I was still trying to determine where the noise was coming from. I set the radio on the other side of the room to one of my Gigabit Ethernet switches.
The noise was to be found across a reasonable sweep of the bottom end of the AM radio dial. I believed the peak to be around 700kHz but now realise the dial is inaccurate, and it's a lot nearer to the magic 612kHz than I first thought.
The noise was very loud, tuning the radio to a pop music station, the music was quieter than the interference.
The pulsing sound you hear coincides with activity lights flashing on the switch, this is why I at first thought the switch or it's own PSU was to blame.
The pulsing is actually a dropping away of the noise, which is otherwise constant.
As you listen to the recording, you'll hear the pulsing stop - this is because I am at this point unplugging the Ethernet cables from the switch. I then try powering down the switch, but the noise remains constant.
There is one point where the sound fades away, this was either me retuning the radio or moving the mobile phone I was using to record the radio as I repositioned it.
I eventually tracked the noise down to a rogue Dell PC downstairs, which was switched off but plugged in to the mains, so the PSU was still "live".
The second recording is one I have just made, to show how that same frequency sounds with the rogue PC unplugged from the mains. Although it sounds as loud as the first, in actual fact I have turned the radio volume up full as I was concerned the sounds wouldn't be heard, they are MUCH quieter than the interference was. To me this now just sounds like typical AM static.
http://sheddyian.hopto.org/interference1.mp3 < Interference 1 (This was heard all around the house)
http://sheddyian.hopto.org/interference2.mp3 < Interference 2 (AM silence once the rogue PC was disconnected!)
Ian