[David]

Many years ago was helping a friend with a sound system that was just installed at his church. When turned on always a buzz I always carried one of those adapters that allowed a 3 prong plug to be used in a outlet with only 2 prongs. No buzz! Over the years several breaker panels had been added to different parts of the many building all feeding into the main service. The new panels all had the grounds and neutrals tied together making for a nice feed back loop. We simply isolated the neutral from the grounds in all the added panels no more buzz. The same problem can happen with a backup generator feeding to a main panel with a transfer switch if the bond between the ground and neutral is not removed in the gen sets, most sets have the ground and neutral together for safety.