I went thru this routine --in making the 94 DB required for pattern flying
the engine - a ZDZ40F3A which is one using a different ignition curve and runs very well in light shell composite models
These types ring like a brass banjo -you have to tap around the model with your fingers till you find and stop any plink plink or rattle
THEN----- a looong pipe and a higher pitched prop - I used the new ZDZ 20x12 wooden prop which is quite a surprise- it is very quiet and the shape and stiffness are different -in comparing with other wood props.
So -when applying what Ed and I found on this model - the same work is required on larger engines /models

rubber/loose or shaky mounts are -quite frankly a double edged sword and I won't use em - and yes I know about the rotating plates and shock absorber types - that is all old hat.
A mount that is progressively distributing the load thru the fuselage is still the best approach - -iTis really "mass balancing "
the front of a fuselage can easily be induced to ring at various rpm THAT is the big problem -and each design is it's own problems to sort out -use longerons or strakes to stop these vibes.