Its been doing it off and on for a while, I just noticed it because the
sound was so hard to localize. Anyway it makes this singing noise off and
on - mostly when I have a load on the system - my computer, the dryer stuff
like that. I describe it as a singing noise because its the sound that
metal makes when it vibrates rapidly - it doesn't do it for long - maybe 1
to 2 seconds at a time.

What the heck is going on? It's a 100A non-grounded GE circuit breaker
box - no main breaker, installed in 1959.