I haven't worked with a durso but I have worked with a bean animal ... in which the secondary pipe is much like the primary pipe in your durso (e.g. the one with the little tube sticking out the top).
It sound like your return pump was feeding water to the DT ... and therefore into your overflow chamber ... fast enough for the main durso pipe to form a near-siphon. And the little tube in the top was too small to allow enough air to be pulled in to prevent a siphon.
The short story: a larger diameter tube in the top of your durso should allow you to run your return pump a little bit faster without turning your sump into a hot tub.
The longer story, here's what was most likely happening:
Since that little tube on top of the pipe wasn't allowing enough air in, your primary pipe was fluctuating back and forth between siphon/not-siphon ... and the while the little tube was letting in just enough air to make the near siphon frothy and full of bubbles.
The result: gurgling noises as it tried to suck air into the siphon ... and yet enough turbulent, froth water rapidly flowing down your main durso pipe to create a frothy hot-tub-jet-effect in your sump.
When you dialed you return pump down to its lowest setting, the flow in the pipe slowed enough for the little tube to be able to draw enough air in to prevent a siphon from being near to forming.
If you want more turnover from your DT to your sump -- e.g. to turn up your return pump a bit -- you'll need a larger diameter tube on the top of your durso pipe so that it can draw in more air quicker. This will let more water flow through before a siphon starts to form.
But it will still mean slightly louder gurgling noise. Some gurgling is unavoidable without either a full siphon, which has no air being draw into which means no gurgle ... or with a very slow flow down through the drain pipe, which allows the air and the smaller volume of water to both occupy the pipe without trying to mix.
See pics.
