Hi Zim
Welcome.
Unfortunately, nitro engines arent particularly quiet and as far as I know there isnt much that can be done to make them that way. Its not so much the level of noise as the irritating cross between an angry wasp and a chainsaw pitch of them. 4 strokes are much quieter but they are also not much use for helis (you can get them but they are rare)
As far as quiet pipes goes Im told the zimmerman pipe is about the best.
Hope this helps... Azzy