Overflow noise

Does the water in the overflow rise, and then "flush" creating the noise? If that's the case it sounds like it just needs some tuning. Do you have the model that you can raise and lower? Another thing to check is the air vent hole. Make sure its cleaned out. On standard durso standpipes you adjust the size of the air vent hole to "tune".
Is the water dumping into the sump above or below the water? If that's where your noise is coming from do a google search for reverse durso. Dumps the water out about an inch down, but vents the air so you don't get all the splashing and noise.

 

Try raising or lowering it until you find a spot where the water level in the over flow stays constant. This will stop the "flushing" anyway.
I would also look more into the reverse durso for down in the sump where it dumps, or at least get some kind of plumbing set up so the air can vent above the water line in the sump. Limit the underwater bubbles and splashing. A simple T connection right at the water line is adequate, but it won't be as quiet as the reverse durso.