There is no reason to drill a hole, 3 inches away from another hole. Gravity is the same, and the water table in the main tank is the same, at the top of the water fall. The placement of this hole would make no difference at all, since the water level within the overflow box, is still near the top, should be about an inch or 2 below the main tank, at the entrance to the U pipe. The bottom hole is actually the best spot to place it, because you can control the water height, but using different length pipes. If you used a side hole, there would be an elbow as soon as you entered into the overflow chamber, which would then point up, right where your original hole is. If I recall, it's impossible to squeeze in 2 bulk heads in the little space, since the manufacture centered the 1 bulk head.
If your goal is to reduce the splashing noise, you need to adjust the length of the straight pipe, so the water falls a short distance. your picture looks about right.
The second source of the noise is the sound falling within the pipe itself. To eliminate that using a single drain would be impossible, but with your set up, what you need to do is drill an airline hole right to the top of the U. You may already have one, I don't remember if the set up comes with that little hole. If not get a PVC end cap from home depot. Once you have the hole, insert a 6 inch air line tube. This would allow air to bleed into the pipe, while water flows in from the U end, and that cuts down on the gurgling. finally, put a silencer of some sort on the air line. You'll chance the toilet flushing sound to bubbling sound.
If you do that, you'll see that the water level in the chamber will be at the top of the U, not in the middle, as show in your photo.
