Late to the party again, but regarding that wire at the ceiling position in the earth terminal....
It looks like a brown insulated conductor. Or is it bare copper?
Can you check continuity between that and a socket earth terminal?
The first thing I tried was the continuity test, but my multimeter wasn't playing ball. Maybe it's not sensitive enough.
Then I tried using my tone generator for tracing wires. I placed it on live and earth. Which worked in the kitchen but nowhere else. Maybe it doesn't work well in that application.
Then I opened up all the switches and light fittings and inspected all the wiring. I found loose wires and ones connected that didn't make sense. There were two black wires twisted together separate from everything else, I separated them and isolated them.
So I isolated all the loose wires, cleaned everything, took notes about what I had done and put everything back together.
The buzz has gone now.
But I am concerned about the electrics, the upstairs light switch definitely has no earth on it.
I'm going to do more research and invest in some equipment to sort this out.