If the water flow through the Durso is too fast, it creates a complete siphon and sucks the water from the overflow chamber faster than the tank water can get into the overflow.
This will result in significant water level fluctuations within the overflow chamber as it cycles from siphoning and then waiting for the water level to reach the proper level to drain again.
The gurgling sound usually comes from the air around the lip of the drain pipe getting sucked in along with the water being siphoned. Similar to when a toilet is being flushed.
To address this, you will need to adjust the size (or number) of air holes at the top of the Durso Cap. The holes at the top should let just enough air in to regulate the pressure inside the standpipe.
Too little air, and you end up with a complete siphon and too much water flows through.
Too much air access, and you could end up with a noisy standpipe as the water freefalls down the length of the pipe.
Start with the smallest drill bit size that you can use, and observe the water level in the overflow chamber for a couple of minutes.
If it is still fluctuating, then slightly increase the size of the hole, or add a second hole.
Don't make too big of a hole at once (or too many) as you could overshoot your target.
You could also use a bigger return pump to match the current overflow setup, but this would be a more expensive option.
By the way, what diameter pipes are you using for the Durso standpipes?
