I have the synergy overflow. If you use the stock pipes that come with it, you will deal with a lot of noise. They are not cut to the correct height leaving room for water to rush into the box making it very noisy. This is why the med pipe length needs to be cut that is just approximately 1/4" below the height of the overflow holes. Else, you get loud rushing water / splashing sounds coming into it. At this height, the water flowing in won't make the noise that people are complaining about.
It is bad design on synergy's part by having the external box so much lower than the internal box. However, with the adjustments I explained, you'll have a dead silent box. Oh, and yes, a good quality gate valve is very important to fine tune it like Hui mentioned.
Also, pro tip, get some teflon tape and put it around the edge of the lid so when you cover it, it prevents any salt creep.