Sump/drainage noise

[exnisstech]

Sounds like you need more air to enter at the top. Drill a bigger hole and it will stop gurgling.

Good thinking. When I ran my single drain hobs I drilled a hole large enough for a piece of air line then moved it up and down inside the drain pipe until I found the sweet spot.

 

[DCR]

The best solution would be to convert the Durso to a Herbie by making the current return a full siphon drain and running the return over the top rim of the tank. If you stay with the single Durso, you will probably have to further reduce the return flow with a gate valve on the return line until it gets acceptably quiet for your ears. The noise is coming from air being drawn into the drain line through the siphon break hole at the top of the Durso and creating air/water turbulence inside the drainpipe. The water flow has to be low enough so that entrained air can easily separate from the water and escape back up through the drain against the water flow.

 

[mfinn]

The best solution would be to convert the Durso to a Herbie by making the current return a full siphon drain and running the return over the top rim of the tank. If you stay with the single Durso, you will probably have to further reduce the return flow with a gate valve on the return line until it gets acceptably quiet for your ears. The noise is coming from air being drawn into the drain line through the siphon break hole at the top of the Durso and creating air/water turbulence inside the drainpipe. The water flow has to be low enough so that entrained air can easily separate from the water and escape back up through the drain against the water flow.

My thoughts exactly.
The combination of the air and water in the drain pipe is the cause of the noise.
Choke it off so only water goes down and see how quiet it gets. But having the emergency drain is a must.