Today I had Comcast come out to check on a problem I was having with my television service; one particular HD station on all of my TV's had poor reception. The technician came and redid some of my splitters and connections in the basement, and while we were talking he mentioned that he made a few little changes on the outside including shrink wrapping the coupling from the street and redoing the ground block. Apparently these are current protocols for any service call.
When I heard him say "ground block" I immediately had visions of a ground loop hum, and I asked him if there was any chance that what he did might cause a problem. He said "No" (of course) as it now had an even better ground connection than it did before.
After he finished up I checked the problem channel on my TV's to make sure it was fixed (which it was), but I didn't think to check my audio system for a ground loop. After he left I turned on my stereo system and sure enough a nice 60hz hum. Wonderful.
I had my amp plugged into its own circuit, so I moved it to the same plug as the rest of my system to try and get them onto the same ground. Didn't work.
I then went to the basement to see if he changed anything down there related to the ground, but it was unchanged; the ground wire comes in from outside and is connected to a water pipe - same as before.
So I went outside to see what he did to the ground block. It looked fine, but I unscrewed the ground wire and saw that he had pushed in the wire too far so that the insulation on the wire was getting in the way of a connection.
I fixed the connection and went inside to check on the hum, and sure enough it was gone.
Nothing earthshattering here, but it just goes to show how easily a ground loop hum can be introduced into a system and how tricky it can be to isolate the cause.