I'm not quite sure what's going on here - the thing that it most reminds me of is the occasional interference I pick up on my house electrical service from a ham radio operator down the street - mumbling speech that my PC speakers will play sometimes and that has on a couple of occasions cropped up in the main system. Is your sub on a separate circuit from your main system? If so, could it be getting some noise from other devices (refrigerator, for example) that are on that circuit with it?