I think what doug was saying is that in some of those wiring jobs they have voltages that get induced from a hot ground from another device somewhere and to see if isolating it from that takes care of the problem. There could also be more than one grounding system that are at different potentials. The biggest danger would be if the casing became hot without being grounded. Then if you got between the casing and a ground, you could get shocked. My apartment isn't one of them, but some homes only have two wire systems and you would have no choice unless you could play by the kitchen sink or in the bathroom where you had a grounded 3 prong outlet.

In any event, that didn't take care of it. I think it is getting induced over the air, because I turned on a fluorescent light and the amp flickered with the light as it was lighting up. I unplugged the guitar and still got it, so it's not the pickups on the guitar doing it.