Maybe he took one of the suggestions and that fixed his problem. Or, maybe what he had was one of those intermittent noises that can cause you to rip your hair out, because you can't find it, then all of the sudden, it goes away.
It could have just been a cord that was "iffy" and when he saw it, he fixed it. Something like that could cause you to shy away from saying what it was. While it's going to cause you problems, you might not even notice it, until you start looking for it.
A plug that has a wire that's not quite solid to the ground, a cheap power strip or even an older 2 prong plug that doesn't have one larger blade, forcing you to plug hot into hot. I've had some of those cheap devices where you can plug it in either way and if you get noise, you just flip it around. It's pretty old, but just something like a lamp that happens to be on the same circuit as your system can do that kind of thing.
I've also found that those spiral florescent bulbs are total crap around any kind of system. Or a Rheostat that controls the overhead lights in a room (for that special mood some musicians want). I once had a rheostat on a room switch and ended up taking it out, because if I dimmed the lights, a nasty hum was produced.
The same for those circular florescent bulbs. When they first came out, of course, I decided to replace the bulbs in my room. I have four bulbs on the walls. Man, did those bulbs sound nasty. And what's really strange is I only had to change one back to incandescent, to get rid of the buzz.
I could end up switching to LED, as they are much better than either incandescent or florescent. They are brighter, take less juice and they don't cause any buzz.