You may have some air trapped somewhere.
Check the inverted U shaped rigid tube connecting both boxes of the overflow. Maybe you placed the U shaped incorrectly, the shortest side needs to be in the box inside the tank while the longest end goes in the outer box, where the sponge is.
Also check your
powerhead or water pump, if due to evaporation the sump water level goes down too much, your powerhead or water pump may be pushing some air through the outlet hose.
My Wet/Dry sump is as quiet as it can be. I need to open the cabinet doors and get my head in to hear the powerhead or the dripping water hitting the fiber at the intake tray when doing maintenance.
What brand and model of Wet/Dry sump filter do you have?
Pepe
Santo Domingo
PS upload some pics of your Oscars!