Keeping everything quiet on a reef tank is an ongoing exercise, sometimes successfully pulled off, sometimes frustratingly unrealistic. I've noticed that as I've added quieter gear to my tank, adjusted how the drains run into the sump, and other minor tweaks, other things I never heard are 'louder' in comparison. The sound of water pouring into the overflow box through the teeth sounds like a babbling brook to me. I cleaned that area meticulously, and it didn't change it for the better. I'm going to accept this situation as natural and focus on other things.