Also keep in mind that your overflows are probably on rated for 600 a pop. If your running anything more than say a mag 7 per overflow, there is simply too much water trying to fit down the pipes. If this is the issue, turn off one of the pumps, or simply attach a valve on the outlet of your pump and cut the flow back until it quiets down drain noise. You really don't need a ton of water flowing through your sump anyways.