In my own experience I never put a valve on the pipes going to the sump, only on my return pumps discharge line. I did it this way as only the amount of water you are returning to the tank with the pump will go over the overflow so all my tuning was done at the pumps discharge end and no restrictions to make noise on the overflow side. I suppose as the water goes through the restricted valve and then back into a large pipe opening it would by spraying the water onto the side of the pipe causing some noise? Is the level of water in your overflow constant or is it surging? My only other idea would be that with the valve almost closed the water above the valve builds up a bit of pressure and then surges through the line rather than a constant steady flow.
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