The water noise from the overflow into the sump was loud at first, slushing and sloshing about, but I was able to adjust the return valves to the tank and that fixed it. No more sloshing or water noises. I bought my megaflow sump used and it didn't come with bioballs, I've thrown some base rock in there where the balls were. Without the rock or anything in there it makes quite a bit of trickling running water noise.
My SEIOs weren't working when I first plugged them in.. or so I thought... thats how quiet they are.
Now that Mag is a regular party favor. It's vibrating against the wall of the sump pretty darned good.
My thought is to put a sponge of sorts on the bottom and sides of it to dampen the vibration.
If I could get that vibration fixed, I think the 'noise' would be VERY resonable.
I have an Oceanic 75g RR tank, MegaFlow sump, ASM G2 Skimmer, stand, canopy, and 60lbs of base rock.
Someday I'll add fishies.