September 2010: While I was trying to track down some VERY bad hash as described here, the use of my FT-847 and HB7CV showed that some strong hash was originating form a house a few doors away.
I initially thought that this house was the source of the major noise noise touched on above so I knocked on the door and explained the problem in my usual way - strong interference - tracking to your house - arcing somewhere - potential fire hazard etc etc.
It was about about 20:30 at night and every light and bit of electronic equipment was switched on so a scout round the house showed up plenty of buzzing sounds emanating from every power socket but I could not identify a particular source. I therefore decided to come back during the day when things were a little quieter.
I soon found the source of the problem. In the main living room, the main lighting consisted of three banks of six halogen down lighters set into ceiling with dimmer switches. As soon as either one of two of the banks of lights came on I got hash on my radio at home. I knew this as I had my XYL next to my 6m receiver listening while I turned wall mains switches on and off.
I explained the source of the problem and the owners were kind enough to let me come back at a later date to investigate. When I did this a few days later I found that low voltage halogen bulbs were in use and each had what are know as 'electronic' transformers attached. These are switching power supply based devices and can cause terrific interference. In this case, I felt that my hash was caused by all transformers rather than just one that had failed (picture below). Just how complicated these electronic transformers are see the picture below.
In the main living room alone there were 20 of these switching power supplies! Even if they were all working to CE specification, the radiation was quite enormous.
After talking with Peter, G3ZSS, I decided that the best route to remove the noise was to replace the low voltage bulbs and their electronic transformers with 240 volt halogen bulbs that did not need transformers. This approach would remove what I assume to be the main cause of the interfering hash completely.
This also turned out to be the cheapest solution as I managed to obtain a pack of three 240 halogen down lighters from B&Q for only £9.30! Replacement electronic transformers (that might, or might not, be better) were £5.50 each from Screwfix. I had to cannibalise the B&Q lights for their sockets and make up some short twin core cable to connect things up. In the end I only changed 12 of the down lighters as the bank of 8 did not seem radiate interference - yet anyway!
Breaking one of the electronic transformers apart showed what might be the cause of the problem with the over-heated resistors and blackened PCB.
All I have to do now is buy my near neighbours a bottle of wine in appreciation!