TLDR 5x to 10x turnover seems to be the norm.
I think the first thing you need to do is identify where the sound is coming from. Is the pump loud? The overflow box? The water splashing into the sump or back into the tank?
IME, "loud" usually means adjusting the water flow so it doesn't splash. For example, I stuck a 45 on the end of my overflow's (emergency/secondary) drain with the bottom part of it submerged. This way when everything is working properly, I don't hear it, but if more water than usual is coming down, it'll still make noise and get my attention.
Now, that's for the secondary/emergency overflow standpipe, but I still have a small amount of water flowing through it (on purpose).
The main drain, on the other hand, is fully submerged and completely silent.
You can also end up with a lot of noise if your overflow isn't 'tuned' right to maintain a full siphon.
Another place you can end up with noise is the return pump outlet. If it's splashing it'll make noise (duh). The trick, at least for me, is to get it just right so it splashes enough to agitate the surface and break up any scum floating around, but doesn't make too much noise.
As for recommended GPH through the sump, I see 5x-10x of your total volume. I think mine is set at about 300gph with just under 50g in the system and everythig has been working fine.
