I agree with Mike in that with the breaker for the light switched off, the Neutral (White) wire is still connected and may be shared with your dedicated theater circuit.
I think you are on track in checking the ground (bare wire) to the light fixture and the light switch, and even the theater outlets. They should all be connected, and the source of the problem is probably a missing ground, and/or a device introducing noise into the ground or Neutral. You could go around unplugging everything on the other circuit to see if the hum quits.
If that doesn't work, you could try removing the circuits from your breaker box one at a time to find out which one may be introducing the hum into the system. To do this you may need to do more than just turning off the breakers one at a time. You probably need to locate the White Neutral wire as well and disconnect it from the panel. When you find the one that removes the hum (maybe the one with the light or not, but start there first), you know it is something on that circuit.
*** DISCLAIMER *** I am not an electrician, but I have dealt with these kinds of issues before. Hopefully others may chime in on if this would make any sense or be worthwhile. It was just an idea I had. Please be careful in what you do in the box (just want to be sure you are safe). Get someone who knows what they are doing if you need help, and turn off the power to the box before working in it.
Last edited by dllewel; 01/11/07 08:00 PM.
