Don't expect the answers here to be definitive.
I had to replace my 5kw Onan, 3 cyl Kubota in an Onan shield.
The mechanic helping me with this project recommended and I went with the Westerbeke 4.5 in a Westerbeke sound shield. Theory was their reputation was for the best sound shield, so quietest result. Again a 3 cyl drive engine. Connected to the original exhaust system so no change there.
The old Onan was quieter than my original Norcold fridge. Once I got rid of that gem, I could hear the genset when running. Otherwise, we had gone to bed forgetting that it was on. The Westerbeke, supposed to be quieter, would never lull one to sleep, though it is quiet enough that some folks have not noticed it running.
I think the secret to quiet running lies with the sound shield, not the engine.
The Westerbeke shield has little protection under the engine.
The Onan had a compete, unbroken tray under.
If I spent some effort, I might quiet that area and achieve total silence.
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Keith
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