I have a 1900gph eflux return running at about 70% .... so probably around 1000 gph with head loss, etc.
I played with my 16" Ghost for a little bit, but was able to get it SILENT. Here is how:
1. Main return in the rear box, make it shorter (I think I cut the pvc around 2")
2. Secondary return, make it taller (top of my U is about level with the top of the bulkheads, this is what sets the rear box water level). I think mine is closer to 4".
3. On the primary return gate valve, turn it up until you have a trickle through the secondary, then SLOWLY turn it about an 1/8 of a turn until you find that silent sweetspot. The trick is to have the least amount of trickle possible through the secondary, I found that is where most of my noise comes from.
When I followed Synergy's instructions, my main was sucking air through their siphon break hole in the rear box u joint. So by lowering that to ensure it is fully submerged, that got rid of a ton of noise.
I am sure everyone else in this thread has solved their issues, but this may help future reefers.