The top of the durso T is capped, and you drill a very tiny hole (something like 1/32" on mine) to create a partial siphon. You adjust the hole size so that the water level is at the middle of the T. <That is when the overflow is quietest .... you might have to experiment a bit to get the right size hole .... but the caps are like 10 cents each. <You don't glue it on until you get the hole right!>
If you inlet hole is too close to the top for the Durso T, you might be able to drop that a bit with a U bend. Instead of the elbow on the horizontal pipe being turned up, you turn that down, and make a U below that with a couple more elbows & pipes. That should allow you to drop the Durso down. To prevent floods when the Durso starts, you must have the top of the T below the top of your box, and the horizontal arm of the T higher than your outlet pipe.
If you don't have room for that, there is another system of quieting an overflow. This system requires 2 outlets. You have one lower outlet that is running as a full siphon. (Usually that is a short standpipe, but in your case, you can have a smaller outlet installed lower than your current one.) The lower outlet is chocked down with a valve so that it is draining water completely submerged (no air = no gurgling noise). You must have a higher emergency drain as the water level will fluctuate a bit in this system, and the 2nd drain serves as an emergency flood relief. <I can't find the original site with the pictures ATM .... I can draw you a pic if you are interested in drilling another hole in your overflow.>