> There is a hum... maybe about 40 - 70Hz. I've always assumed it was coming from the power grid – it's the same frequency range, and gods know what metallic s

> There is a hum... maybe about 40 - 70Hz. I've always assumed it was coming from the power grid – it's the same frequency range, and gods know what metallic stuff in the walls could vibrate in resonance to it.