Hi, I'm experiencing some sort of low randomized rumbling noise from my subwoofer (Sunfire True Subwoofer EQ). The noise is present even after Ive disconnected the sub entirely from the amp and place it far away from any other electrical equipment. The only way I can get rid off the noise is by turning down the volume control on the sub to minimum. The noise sounds like someone is scratching lightly with their fingernails on the membrane. I thought that I had rats in the attic...Otherwise the sub is playing fine, no noticeable distortion plenty. What am I experiencing? Is it broken?