This is all very interesting even though we don't have noise regulations at our field. Believe me, I appreciate my luck every time I fly and I never take it for granted. I used to live in Europe, so I know how much you guys love to fly and the rules you have to live with. Maybe it will lighten up one day.

The tight wing fit with a soft seal suggestion is true. I found out with a plane that I could hear in the air. Tightening down the bolts hard made a lot of noise go away.

Covering makes drumming and high pitched sounds. Shrink down your covering before testing for noise.

Control rods vibrate a lot. I had one plane that buzzed loudly at certain high RPMs. Wood dowel rods vibrate a lot. You can isolate all rods when building the plane.

I was a helicopter pilot for years and the big noise factor for them is the blade tips. They approach mach speeds. On the particular helicopter that I flew, the tail rotor was the loudest sound when heard approaching from a distance because of the RPMs. We were taught that the Blackhawk has swept tips for noise reduction. A smaller diameter prop tip will travel slower than a larger diameter, so a slower tip may help a lot. Smaller props make sense if the higher engine RPMs don't get you...unless you use 3 or 4 bladed propellors. It sounds like you need to make a study by experimenting with sharp tips, blunt tips, and maybe rounded tips. You may find the right power/noise combination for your plane.