One of my overflows were making *terrible* noise. When I realized it was only one of them I looked at the two to see if I could figure out what was different enough to cause the noise difference. What I found was that the quiet overflow was closer to where it dropped into the sump and the other one was further away. Since the further one had a straight run at the wet/dry it was running faster than the other one. The closer (quiet) overflow hose had been curled around to get back to the inlet so it had a circle in the hose (going up into the air), and then into the inlet to the wet/dry. When I put a U into the (noisey) hose that had the straight run to the wet dry it quieted right down. Seemed that little bit of bend in the hose was enough to slow down the return enough so that it was always full of water, thereby eliminating the noise of the waterfall into the overflow. It has made us all very happy!
I had originally had a mag9 in there but the noise was really bad. Put in a mag 12 and it wasn't quite so bad but still noisey. That's when I checked the hoses. Seemed in my case that faster flow into the tank and slower flow out was what fixed my noise problem.