I really need help. (Hopefully with a Herbie style drain)

[rocketlily]

I am not having any luck with trying to do plumbing. The tank is 90 gal and I have an over flow compartment in which I have built a durso. The noise of the water entering the sump is unbearable. Something like running a bathtub. Massive air bubbles and alot of noise. It has nothing to do with the holes in the cap, as it doesn't make any difference whether the cap is on the durso or not. I am also getting alot of fluctuating height in the water in the overflow, up and down. Not to the point where it sounds like a toilet flushing, but it rises and lowers.

The tank is in the living room and I really need to try to make it quiet. Any help would be very much appreciated.

Thanks