I had a similar problem with a return that was mismatched with what my in take overflow/return pump was pushing, and here's how I solved this. Think of this as a delicate balance of air in vs. water siphoned. More air in = less water siphoned, more air in = more toilet sounds. No air in = max water siphoned via gravity, which is the max flow you're able to get out of your bulkhead size. I have a 1" myself. For my durso, with a bigger hole, I had terrible sounds and the durso siphon wasn't handling enough water. I found that the solution was actually a smaller hole, which allows less air in and increases the siphon to handle more water. Here's how I dialed my Durso in, I think the instructions they give on the site aren't the easiest way and you'll end up blowing through alot of caps before getting it correct: 1)Take a brand new PVC cap, use a nail(i used a very small one, the type that is used to secure baseboards in houses) and start pounding it into the cap. 2)Pound the nail in until it just *barely* pokes through. You'll slightly notice a hole by looking through it and be able to see a pinhole of light. 3)Cap your durso with this, see where the water level falls. It should in theory fall somewhere between the bottom of the 90 degree elbow and the top of the cap. When I did this mine fell about 1 inch higher than the bottom of the 90 degree bend. 4)If you desire your water level higher, like I did, Put the nail back into the hole and give it some gentle hits with the hammer, this will make the hole slightly bigger and raise the water level and decrease the siphon. Repeat step 3 again. Right now I actually have my water level adjusted to the point where it comes up to the bottom of the cap. It's whisper quiet, and because my stand is tall enough, water falls only about 1 inch from my weir to the water level inside of my overflow, this is also whisper quiet. The only noise my overflow makes right now is the splashing of the water landing in the sump bubble tower, which isn't much at all. Hope this helps.