I've worked on a few, and I believe the Dometic Turbo series is probably the quietest. So long as the compressor is isolated on the rubber feet it comes with, it is nearly silent. Also double the flooring if it is like a sound board when you rap you knuckles on it.
Most of the noise comes from the air moving through the ducts. Keep them on the large size (but not greater than specified--velocity is needed for air mixing in the cabin) and avoid throttling with dampers. Also, try to separate the air return grill from the compressor; that is a noise pathway. Use sealed insulated duct (helps with efficiency, noise, and condensation). If the duct is fiberglass or metal, consider lining it with self adhesive closed cell neoprene foam, for both insulation and sound damping.
I believe there will be a comprehensive article in Good Old Boat Magazine in the March-April edition (yup, that's me)