Intermittent Hum Problem

This is making me crazy: I have a hum that starts after monitoring system has been on for a while. At some interval it just comes on - nothing has changed as far as I can see. If I unpatch the input to my monitoring system the hum goes away. But it comes back with whatever is patched in, so it can't be any source in particular. I thought it might be in my patch bays because it's always gone when there's nothing patched, but it's not always there when something is patched. And to make matters worse, it comes and goes without there being any changes to the patching.


Ideas?

Thanks

Can you describe your monitoring chain?

Is it a loud, speaker-shredding hum or is it a "there might be a ground loop or bad connection" kind of annoying but not deafening hum?

Can you get it to come and go by whacking the amplifier?

Some time ago I has a hum that would come and go in my NS-10Ms, and the problem was some bad filter caps in my power supply.

Well, I hooked a clip lead from my studio ground buss to the chassis of my White 4700 EQ and it got a lot better. I need to investigate some more.

Hmmmm...

I've had problems with Whites picking up hum interference. I've wrapped them in (expensive) mu-metal foil and solved it before now.

Does it hum with the Whites removed from circuit?

Keef