This one caused an intermittent blackout on all HF bands in my shack over the past half year or so. Due to the intermittent nature it was extremely difficult to track down. This morning, it was again destroying all the fun in my shack so I decided to go out with the Sangean radio in another attempt to find the source, and I was lucky today! The source turned out to be an ice-cube machine in the pub next door. A motor actuated microswitch was sparking at a certain point in the ice-making cycle, you can hear the effect of it in this video. You can see the switch and drive motor in this picture. This failing switch also poses a fire hazard so I advised to take the machine off the mains until it has been repaired. I made a dance of joy this morning having finally rooted out this long lasting annoyance!