YEa I am having the same problem, since my tank isnt reef ready, i am using an overflow with U-Tube and its plumbed with PVC to my sump...I fixed the sump noise by cutting some egg crate and wedging it by the water line in the sump and putting some poly pad so when its drips down it doesnt make anymore noise but the overflow is down right noisy so i usually place a small piece of poly filter covering half the pvc outlet where the water goes down it usually works for a couple weeks than gets noisy again (which means its dirty lol) but i havent found a real solution....
They make those insert filters for the overflow but i have a 1" pvc hole in the overflow and i havent found one that fits .....