It's just way too loud...

[Abbyreefer]

I have a dual overflow and have a mag 9.5 on mine with 2 stockman standpipes, when I only had 1 it mad the same gurgling noise because it couldn't keep up with the flow and was trying to push it down.. Now with the 2 stockmans its super quiet... I would say either put another durso (if possible) or just turn your mag down.. Turnover in the sump should only be about 5 times tank volume so your mag would be turned to about the half way point or even less depending on your head loss if you use the above calculations.