The quietest conventional gensets I know of are studio gensets. We used them in the 9th Infantry Division (Motorized) a long time ago (painted proper colors, of course). They were extremely quiet - drove the OPFOR nuts at the NTC because they were used to homing in on brigade CPs by listening for the generators (they never did find our CP - that was just one reason).

Too big for residential standby use, of course - but if you have an opportunity to study one, it will give you a lot of good ideas about how to significantly reduce the noise signature of any genset.

But an air-cooled genset is not going to be capable of being silenced as much as a water cooled one - no way.

Tom