James

A filter / silencer on the air inlet will help muffle inlet noise. Carbon / aluminum spinners "ring" when the engine fires and can be quieted by filling with expanding polyurethane foam. Both will be more noticeable to a meter than the ear. Lining the interior of the cowl with sound reducing rubber (available for instrument consoles) also helps.

Lowering prop tip speed will lower noise, lowering rpm and / or prop diameter will accomplish this. Prop material is another item to investigate.