At the start people where commenting on the monster pump and you posted quiet. Realize now it's plumbed but what's really changed, how where you running it on the test, what was it sitting on, was the fan pointing the other way, etc?
If you can't get it quieted down don't abandon your plumbing but look into what might drop in it's place. I've got a Dart (same make, sort of) and a close fit (2" in, 1.5" out). It's not whisper quiet but can guarantee way more quiet than yours. There's also the Snapper.
Quote from a
marinedepot page "
Quiet Sequence pumps installed near areas of relaxation and entertainment generate minimal noise levels For example, the 1/4 H.P. unit has 52 dB noise level at 10`. A normal conversation is 60 dB for reference." And the Dart is 1/8HP.
If you can try to hear one though first. I offered a reefer in Edm to come over and hear mine, I sure someone in Van would do the same.