I've found another one. This one works the best so far and it was super easy to install. Check out the website: Durso Standpipes Portal > Home
Check out the popular mods section. You can do what they suggest for an overflow box but I just did like the picture on the first page. Bought a T, inserted the 90 and put a cap on it with a hole.
Voila! No more sound. You'll have to adjust the return flow because this does slow things down just a notch but it's no big deal. Put a valve somewhere in your return line and that's it.