Yes, I use the airline tubing. I don't have a gurgling sound. My problem (as small as it is) is the water fall sound from the water coming from the tank into the box.
I've put small pieces of live rock into the box to help with the sound but this did not work either. I may try putting a lid on it as recommened before. Who know's.
I had the same problem with my CPR overflow until I leveled the unit. I had is so it was no plumb to the side of the tank, the water on the output side of the box was hitting the bottom of the box rather than flowing over the lip and down the drain. See if your box is level and plumb, you may have to make it stick out from the aquarium more.
I have a cpr 150 and it came with a lid on it. you need to have the lid to keep it quiet. I was told by cpr to have the in tank side slightly lower than the outside. Also in my case my sump is 15 ft below overflow box so I have it going directly into a bucket which is slotted so water flow out of it in a certain direction. The bucket seemed elimate alot of the air bubbles in sump. I also ran a air line tube from overflow to a maxijet 1200. This works o.k., I still need to figure out a way to elimate air in overflow, I've thought about increasing the return pump size, right now its moving at around 900gals an hour and the cpr150 does 1600. Therefore the need for a venturi on the overflow.
Why not try a gate valve.The unexperienced have expressed flood worries but I can assure you there is not aincreased chance of that happening.I've been using them on my CPR overflows without incident for 3 years.You just need to restrict the flow enough so that the gurgling stops.A ball valve will not suffice for this task as it cannot be as fine tuned as the gate type.A quick fix would be to place something on top of the CSO that will completely cover it.Like a towel or a piece of plexi etc.