Two places where the noise could be coming from - splashing of water flowing into the overflow and noise from the durso drain.
Splashing from the overflow will get better if you raise the water level in the overflow. What size pipe and how much flow do you have? Is the water level in the overflow constant or does it go up and down? Dursos have a limited capacity to handle flow quietly. If the flow is too high they start splashing, then gurgling then flushing. You can tune them somewhat with the air tubing and increase the quiet flow capacity, but eventually they will get noisy.
Was it previously quiet? Have you checked to make sure it's not occluded with algae, snails etc?
I it's more of a flushing sound it's probably the Durso.
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