Very nice looking system.....but sorry, I can't really help. I'm looking, but can't really decipher what I'm looking at. If you are hearing alot of gurgling, all I can suggest is make the air hole in the "r" bigger.
I guess I looked at your join date, sorry. No I haven't solved it but I was thinking if I drilled my hole at the top of the (r shaped) drain I would have better air release just curious if it will work before I start drilling holes.
I did try it. It seemed to make more noise. I than got some advise from reef octopus; they suggested that I cover the hole with silicon, then slowly make the hole bigger. This has seemed to help; there is still a little more noise than I would like but I think this is what I am left with. Interestingly, as I covered the hole the water level decreased until it made a noise like a toilet flushing. As the hole got bigger, the water level increased until the water was 1/2 covering the round part of the r, this is were my tank is the quietist. I would suggest that you play with the hole size with some bluetac or tap (you could even use you finger). I used my finger to try it then I went with the silicon. Hope it helps!
You can use an air valve and adjust the amount of air coming in. Just drill a hole large enough to thread in an air valve with teflon tape and then open all the way and start closing it until the noise stops.