I just noticed something while I was calibrating my sub. When the pink noise from the internal receiver test-tone (denon 3802) is outputted to my sub, it not only produces bass, but some weird resonance noise (a bit like a geiger counter...) coming from inside(!) the sub. I don't think it's the driver. First I thought something was shaking due to the bass, but it wasn't that. Is it some electronic noise? I put in a dvd after that, toned down the other channels except the sub's, and listened again, but the bass sounded fine... What could this be?!