Your vent hose is to large in Diameter. It's not matching the drain size to the cap hole, but matching the cap hole to the flow requirements. I have 6 0.018" holes in my cap for venting, about 300GPH drain. and it's silent. Try getting a shutoff on the vent, something you can precicely throttle back untill it's silent.
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Dan Pesonen
Umm, a tank or 5
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