I would just start over and make a new one and don't glue the end cap on and start with a small hole and mod as needed if you make it to big change the end cap. it's simple to make... h/d... lows. It's all good.
I would suggest using larger diameter plumbing. The plumbing in the pic looks pretty small.
Even if you have to choke it back down at the very bottom to fit in the bulkhead it may solve your problem. FWIW, the Mega Flow plumbing kits use this approach.
<<edit>> after looking at the pic again it looks like you may already be using the Mega Flow plumbing kit? If this is the case then you can throw my above idea out the window...
were is the noise coming from? you have the foam on the nozzle side is it from the cut out being so big (looks modified or screwed up) that its letting more water into the overflow than it can handle thus making a flushing noise?need better picture's tom.
your pipes should be 1 1/4 reduced to 1" right before it hits the bulkhead
Start with a 1\8" hole in the cap and see how that works. If you do not have a cap on the duros it is like having not having a durso.
A little info on the water height in the overflow once you have the design down will help.
Fluctuating? very high? very low? sluping gurgling? flushing?