I like what Fabrizio said: *) The engine ist not the cause #1 for noise problem: check for noise emission on retractable landing gears, control surfaces, canopies, etc. On a "sloppy" plane this can make out as much as 4 dBA, that's quite a lot! Of course this could not be an issue with most of the better built planes. Eliminate all noises not coming from the engine or exhaust - this is very important! Watch for any type of vibration (also from the inside)
This is what most people over look. I have used carb filters (little metal ones with mesh) on my OS's without compromising power. Not sure if it makes a big difference in noise though, but every little bit. I also used to put a bicycle inner tube over my hatori pipes to eliminate the metal *ting* from it.
One thing I did see most of the Japanese and a few Europeans doing was taking some foam (like the YS gray foam) and slicing it to about 4mm thick and gluing it over the plug and header holes on the fuse and then slicing it like a pizza (plug hole) so the igniter goes in. Also might want to put some foam on the belly pan surface where it goes over the wing.


Jason