So is that the drain and return drilled high up? If so, which is drain and which is return (looks like return is high and drain low)? Are you draining straight through the bulkhead, or is there something inside the weir area I can't see connected to the bulkhead? What size piping? If the drain is really high up, you likely won't have room, but a DIY durso standpipe is one option for silencing the drainage. A stockman standpipe takes up a little less room, and can also provide a lot quieter operation. If it is constantly gurlging and draining only through the bulkhead (no other fittings), one of these options can reduce the sound for you.
If there is an issue with everything not draining properly, there are several things to look for. Since this is a new system, the drain line could have air in it, which will reduce the water flow until the air gets knocked out of the pipe. Sometimes, blocking the outlet of the tube with your hand, then removing your hand a few times can push the air to the bottom and out of the tube. Return pump sizing can be a problem, too. You only want about 8-10X turnover through your sump, generally (other turnover rates added through skimmers, circ pumps, etc.). A pump with around 600 gph (accounting for head loss) is roughly what you'd want.
I saw that you mentioned flex piping, then hard piping. If you get vibration, using flex piping at the pump or equipment connections, then going to hard piping can reduce sounds from vibration. The flex absorbs a little, reducing the rattle.
