I had to do pretty much the same as Bluetang, I run a mag3 on a 27gl and I was getting a siphone as water was drawing too fast into the sump from the overflow which caused a slight backup which led to a loud flushing noise everytime the intake line drained into the sump. To improve this problem(5 years ago) I also installed a ball valve on the intake line half way down before it reaches the sump. I simply turned the valve back slightly to create a full intake line, and now no flushing and no my overflow does not backup.
I had this problem due to two reasons, one my intake to the sump feeds directly underwater into the sump wich created a flush effect and also because I used a 3/4 bulkhead instead of a 1" fitting. Live and learn
Goodluck