I might suggest moving the CO2 line back to closer to the original location. That will likely require cutting the "tee" off the body so much of the parts would be gone at that point. I would guess that the CO2 being put in through the large fitting lets it collect in a large bubble there before getting into the water. The diameter of the pipe seems more than 1 1/2 inch? Does this let the incoming water fall and splash inside the reactor? When the CO2 line is pulled through the side wall and ended in the middle of the water flow there is less chance of this and noise is not a problem. I use several built to the original desingn on 75 and 125 gallon tanks and they make no noise.
The problem would seem to be that you have moved so far from the Grigg's design that it is unfair to call it his design. There were many reasons for using things the way he designed them and when people "improve" the design some small points are lost. I would suggest the easiest way might be to start over and build it as designed.
An easier way to start might be workable. If you tilt the reacor so that the large space on the side formed by the tee is down, it might keep the large bubble from forming. Try filling the reactor before turning on the CO2. Might work/ might not?