I've found that the sound occurs both at high and low flow rates. I have a Jebao 6000 l/h on a 50gal acryllic (46 actual gal). I can run it on setting #1 or #10 and there is no difference. I'm happy that the box can handle that range of flow, though. Very happy in that regard. It's amazing how much flow you can get from a 1" vertical pipe.I even thought about building up the outside box another inch so I can raise the water level. Might go that route if this new pump I get doesn't work.