I must assume from some that have posted here no one that lives in an older home that has only 2 wire receptacle outlets, (no equipment grounding conductor), has audio equipment that has three wire plugs.
My home was two prong plug when I moved in and never had a noise problem. Over the years as my AC was upgraded and the three prong plugs of audio gear were actually connected to ground, I began to have problems.
When I say problems, realize that having done photography for various audio manufacturers and reviewed equipment for PFO as well as my audio groups testing, I've had a heck of a lot of equipment connected here.
Ralph is right that grounds are done many different ways on various pieces of audio gear and sometimes all you need do is change preamp or amps and the problem pops up.
No one has mentioned electrical phase. Many stereo systems are connected to both phases (or sides of the supply transformer outside) with ground shared. Good way to get hum and very difficult to get rid of.
Again, equipment dependant. I'm sure some of you are connected this way and have zero problems.