I just used the standard durso on all the tanks I've owned. I would say mine is relatively silent. Like stated above I had to enlarge the hole. I also believe it had to do with my return pump and volume. In some of my initial tank setups, I had a substantial amount of flow through the sump, thus the overflow had to keep up with this. More flow, the louder it was. After reading a few articles on Melevs site, I went with the recommended ~3-5x flow through the sump. As a result of that, the overflow wasn't as loud as the volume was minimal.
What return pump are you using on your 40 gallon?