The overflow in my tank was also very quiet, with the noise of the water entering the sump way too loud. I had a 1" return line from the pump and a 1 1/2" drain, with a gate value to control the output. I was told by co-workers that I was probably creating a vortex at the gate valve because the drain was to big. I then reduced the drain size in the overflow to 1". There was a considerable reduction of noise at that point.
After that, just for fun and because I had it, I attached a Tee to the bottom of the drain, below the water level. I don't know why it worked but it made a considerable reduction in the noise. The Eheim pump is now the noisest part of the tank.
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