Due to your hole being quite low in the glass you need an overflow box. Otherwise modifying your drain so that it's completely submerged will cause your tank level to drop greatly. By doing this you can use either a durso in a similar fashion to what 'newguy' said, or you can use a gurgle buster as you will be able to have it lower than your tank level so that it won't flood over. This is the reason that all drilled tanks have overflows, if they didn't need them they wouldn't have them. You could also try cutting your flowrate through the sump way back and adding an additional powerhead to the display to make up for the loss of flow. The less water flow through the drain the less the noise 'should' be.
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