The europeans have been devling into making quiet engines for the last decade...

Basically, there are four rules to making an engine quiet..

1. Prop noise... try using a scimitar prop blade (i.e. apc) and swing a little more diameter than you would normally, this keeps engine revs down and stops the noise associated with supersonic tip speeds.

2. airframe vibration. Simple fix here, soft mount the engine, either buy building a soft beam mount system or buying one of the proprietary items from your hobby store..

3. Exhaust noise, you might want to try one of the proprietary after mufflers, or something like that, or failing that, facing an exhaust deflector straight up will kill a fair whack of in flight noise..

4. carb noise. This is one that is forgotten time and time again... The europeans actually started to make carb ducts which sucked air over the top of the head, not good for performance but greatly reduced the db's.. intake filters and the like will make some difference... but you really need to make sure that the carb is not out in the open or the induction noise will be the killer..

Enjoy