IMHO I think I have found what may be the closest to the real sounds for the FS Northwest Bell 47
http://www.hovercontrol.com/cgi-bin/ifolio/imageFolio.cgi?direct=FS9_Helicopter_Soundkits
Over at hover control I D/L the file B47G2Piston Kit.
It resembles the Lycoming O-435 piston engine.
Granted it is for FS9, but hey, it works well with the FSX version.
http://www.hovercontrol.com/cgi-bin/ifolio/imageFolio.cgi?direct=FS9_Helicopter_Soundkits
Over at hover control I D/L the file B47G2Piston Kit.
It resembles the Lycoming O-435 piston engine.
Granted it is for FS9, but hey, it works well with the FSX version.