if it is still making noise after the elbow addition and you still are throttling back a MAG 9 I would say the hole in the cap on top of your Durso is too big. I rectified the problem with a mod...it was mentioned earlier, the John Guest valve to control air flow.
I also had to cut back the elbow due to size restraints. Also I added a snail trap, believe it or not I did get a snail over my overflow and into the standpipe. What a mess.