1. The hole in the Durso controls the amount of air & so the noise. You need to experiment a bit to find the best size. The size will need to be adjusted with any change in the flow rate (eg a new pump.) <Generally, it is quietest when the water level in the overflow is around the mid-point of the T inh the Durso.>
2. Restricting the flow of the pump (with valves, constricted tube) stresses the pump & wears it out. It is better to put a T in the return line, with one arm of the T returning to the display, and the other arm diverted back to the sump. You control the amount of water diverted with a valve at the arm that diverts back to the sump.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.