SebM:

Propellers get noisy mainly when the tips reach the speed of sound (M=1).

Try the combination that produces the lowest tip speed, which means lower rpm for a 11" diameter propeller.

I suggest using the 46 engine with a 11 x 6.

Check these interesting articles:

http://www.lcrc.org/stu22.htm

http://findarticles.com/p/articles/m...g=rbxcra.2.a.2

http://findarticles.com/p/articles/m...g=artBody;col1

http://findarticles.com/p/articles/m...g=rbxcra.2.a.2

Regards!