please give some advice my rsw12 starts a deep rumbling after it is on for about 15 minutes or so i have unplugged the unit and unhooked the sub feed from the amp to the unit turned it back on and i still have the same loud rumbling noise when it is just plugged into the outlet and the on switch to the on position. if i leave it go for a few minutes it gets louder and then seems to shut off on and internal protector. any ideas????