WOW it must be really quiet where you fly. OS 91FS engines are pretty quiet.

I found a noticeable difference in perceived noise when I switched to an APC prop. Also, a 3-blade of slightly smaller diameter would be quieter because of lower tip speeds.

I have see guys who ran a rubber tube from the exhaust to keep the slime off the plane..but that would damp out the exhaust noise, too.