Interestingly, microlights (which used to the source of many noise complaints) have to meet maximum noise output requirements. Nobody has ever imposed this upon light aircraft.
CAA fees, last I looked, were £61 for a minor mod, and £294 for the first 3 hours of looking at a major mod, and £98/hour thereafter. But the information that they require for approval of a mod is fairly stringent.
That often shouted-about incident where the CAA wouldn't approve a silencer on an SLMG was basically down to the hushkit manufacturer failing to provide the design information that they needed.
Personally I think the main problem is pilots who seem to feel happier at 1000ft instead of 3000ft so much of the time. I see them regularly in the village in which I live, where a higher altitude would be safer, easier for navigation, and clear of controlled airspace - yet they still insist on mucking about under 1500ft when there's no particular need. Also, why fly over a village when you can fly around it?
G