I have an 8" Velodyne powered sub. When it is turned on there is a low pitched (duh!) fairly low volume pulse/beep that comes from it. Perhaps 10-15s between each sound. It does this when the sub is hooked up to the amp and when the line in signal is removed. So it clearly is the sub. Is this something we just have to live with? Or is this a fault with the sub? Thanks!