The hun you are hearing from that box isn't from EMFs. It could be a lot of things though. A common one is a high pitch hum from transistors. That can actually indicate that they're wearing down (kids can hear this the best. So ask them about a high pitch noise from an old TV. They'll think it's normal). A low hum could be something mechanical inside. Or is could be some kind of resonance. Hard to say what they are, but they aren't EMFs.