Treatments to reduce generator noise
Usually generators are in the engine space, which should already be well sound proofed. If the generator is in a different space, acoustic insulation should be implemented, following the same guidelines you would use for engine room insulation. The low frequency nature of generator noise makes 2″ 2lb Vinly/Foam Composite the “best bet” for generator spaces. The insulation will block the airborne sound created by your generator. This will also augment the sound shield that is optional on sound shield available with many marine generators. 
Because generator typically run at rest exhaust noise can be bothersome on deck, especially in a quiet anchorage. Soundown’s WaterDrop exhaust silencers will reduce or eliminate generator exhaust noise ensuring peace and quiet for you and neigboring boaters. Details on generator exhaust systems and related options are available here.
Structureborne noise from the generator is caused by vibration of the generator that is transmitted to the hull and affects all areas of the boat. The most efficient way to treat this path is double mounting the generator. Double mounting is accomplished by adding a secondary set of vibration isolators below and heavy base below the drip pan to which the generator is mounted with factory supplied primary mounts.
One great indication of whether the noise is structureborne or airborne lies in how much it dissipates – if you can hear it from further away, it’s probably structureborne. Airborne noise is blocked by bulkheads more effectively than structureborne noise dissipates with distance. If the noise level two cabins forward of the generator similar to the noise level in the cabin adjacent to the generator it is probably travelling through structure. Feeling the hull or generator is less effective – the fact that you can’t feel the vibration doesn’t mean it’s not structureborne noise. Audible spectra are different from the vibrational wavelengths we can feel. So a generator can produce structureborne noise that you won’t be able to sense by touch.