You can raise the height of your durso so the overflow is full of water. If the water only falls 1/2" from the tank to the overflow it's going to be much quieter.
Make sure your durso is built correctly.
You can add a ball valve on the overflow line to create some pressure on the line. Depending on the distance from the tank to the sump, the extra pressure could quiet down the water noise.
Make a vent for the air (caught in the overflow line) at the surface of the water of your sump. Less bubbles=less noise.
Go skimmerless, or run your skimmer part time. Like when you're at work and at night when you're not in the room.
Research noise ratings on pumps before you buy them. I bought a Mak4, then fired it up. The screaming noise that came out of the pump made me sell the pump the next day.
Buy different manufactures of pumps. When my tank had 3 Eheim 1262s running at once, I could feel the vibrations across the room. They all ran at exactly the same speed, so they were 'in tune' with each other. Two pumps were more than double the sound of one. Three pumps was a definite hum.
Change out your noisy fans for quiet ones. Again, check the noise rating before you buy. There are (nearly) silent fans. Pay the extra $2.
Best advice? Remote sump in the basement, or behind in another fish room. I never thought a reef tank could be so quiet until I
saw a tank with a remote sump.
B