[Myka]

Where is the noise?

When you say you must crank the flow down to reduce noise then that sounds to me (har har) like maybe the water level in the overflow is too low and you're hearing the water falling into the overflow??

If the noise is coming from the pipes - gurgling? - then there is likely air caught in the pipes. Horizontal runs will do that because it slows down the water which allows bubbles to collect.

How much horizontal distance is there from where the water exits the tank to where it enters the sump? How far under water are the drain lines submerged?