I haven't read back far enough to know exactly what you have set up, but if you have a gate valve on your drain pipe you can close it until it runs completely full and sucks no air. You will want a second drain that can allow for some fudge room... if the water isn't draining through the main drain quickly enough it can go down the secondary instead.
Here is an example of my overflow. The main drain is the one that is completely submerged with the black triangular screen on it. The next drain over is the secondary drain which usually has a slight trickle of water going down it. This drain regulates the water height in the overflow. The last drain on the right is a straight up emergency drain and is capable of handling all the flow if the first two drains are completely clogged.
Because the main drain is adjusted with a gate valve, it is completely full of water and the end of it is submerged under water. No air goes in so none comes out and it is completely silent. The trickle in the secondary drain is hardly audible as well.