The predominent source of noise from this engine most likely is the prop. Use the largest props recomended to keep the RPM down and consider mounting the engine on elasomer isolation mounts to keep the engines vibration out of the airframe. APC props seem to make less noise than the square tipped Master Airscrew props do. Pusher props really wail.
I ran APC 11x6 props on a OS .46 FX that was mounted on Du-Bro iso mounts. This plane could almost have passed for an electric plane while in the sky. If someone else was flying one of those noisy four-strokes, my plane would be completely drowned out.