I've read a lot of posts about full-siphon being one of the main ways to make a durso and the overflow weir quiet, but what I'm hearing from all of you is that I need to change the water level (with the vent open to the air) so that it is not full-siphon.
Yes, but all the safe full siphon drain systems use secondary open channel redundant drains. Thus my suggestion earlier to change yours to a dual drain herbie. That’s what I did when I ran marineland tanks. This is probably the most sensible explanation ......
Herbie Overflow Plumbing Guide for Quiet Reef Aquariums - gmacreef
The Herbie method can quiet down your aquarium drain and eliminate microbubbles. The guide includes helpful diagrams and explains each component.
gmacreef.com