All right seeing we are looking at all kinds of causes, I had an induced noise from a power pac plugged into the back of a piggy back plug (or double adaptor) the item with the power pac was fine but the other piece of gear, in this case a Yamaha SPX90, suddenly started to hum and have noise problems.
I also had problems in one of my early studios with the telephone exchange across the road, I was getting induced noise into the mains supply cables, this was extremely hard to get rid off and the only way was to use very good quality cables. It caused real problems whenever a single coil guitar turned up as that picked up the airborne stuff.
Just pointed this out as there is sometimes no simple answer and investigation is needed.
Go back to my answer #8 where I asked if the power point had a good earth, don't assume it does, even the house earth may be bad or non-existent. I did move into a house years ago where someone had broken the main earth cable through the roof between the main switchboard and the main earth point, just left it and did not fix it, a deadly trap for some poor person later. Fixed that and put down a better earth stake and all noise including the stereo in the lounge went away. Check the earth resistance in the house!
Alan.
Very true. In homes these days, we can't assume anything is done to code. Home owners can be the cause of doing something incorrectly, because they didn't want to call in an electrician, because of the cost. Some also assume they can do just as good a job. But, I've seen cutting corners in places that will make you scratch your head. I was asked to install a new light switch in my sister's front bathroom.
I first had to check to see which breaker the switch was on. Her box wasn't even marked. We fixed that issue. Then, I found that that switch had not one, but TWO hot wires coming to it, so if you cut the breaker it was suppose to be on, there was still electricity going to the switch. It wasn't going through the switch, but just sort of pathed into the wiring behind it. That was so strange to find.
Her house was the show house for the area houses, when they were all new. The contractor lived in her house, while he was building all of the others. Being the first one finished, maybe he learned how to do some things with that house. There were other troubling things in there, too.
Her living room was a very high cathedral type ceiling with beams and decorative wooden slats. She got a new roof put on, after a hail storm. They didn't remove the old roof, but just used longer nails to put on the new roof. That's when she noticed that the nails were going through this decorative wooden slats of her ceiling. She had lived in that house for many years and just then she found out there was no insulation in the ceiling of that room and the thickness of the roof there was less than 2 inches. Talk about cutting corners!
So never assume someone else did their job. You could have earth ground wires in your outlets and they could be connected as they should be in the outlet itself. But, if they aren't properly grounded, they are just a mess of green wires going nowhere.