My 2,800 rpm NextGen 3.5 with its sound enclosure and inside an insulated engine room, makes very little mechanical noise. An engine compartment ventilation blower makes more noise.
But the exhaust has an annoying putt-putt sound. This is a one cylinder engine and it is hard to differentiate the effect of 2,800 and one cylinder vs 1,800 rpm and typically three cylinders firing, but it there is one. I will bet that a sound meter would show little difference behind the boat between the two. But the sharp putt-putt is definitely more annoying than the low rumble of a 3 cylinder, 1,800 rpm machine.
But a 3.5 KW NextGen is probably half the price of a 5KW Northern Lights, 1,800 rpm generator. And with the way we use ours, I would never consider paying the difference (if it would fit!) for the 1,800 rpm genset.
David