The other day I noticed some rumbling coming from the sub (stadium III) It's left permanently on but was not in use at the time and the amp was off. It was as if there was a loose connection causing feedback. I checked all the connections, they were all ok and I could not recreate any noises by moving them about or unconnecting/reconnecting etc. It was intermitent and only lasted for a few minutes. I think it has happened before but I have previously put it down to my kids tv/music upstairs. I may be clutching at straws here, but it coincided with the onset of some stormy weather. Could some kind of static electricity have caused it.