I have recorded the sound of the 2011 Yamaha XJ6 600cc at GTMax Shah Alam. The salesman there, Hafis are very helpful and asked me to try to listen to the engine sound.
The sound of this XJ6 are really soft and almost silent. It sounded very much like a very precise machine in action.
I think you just need to listen it by yourself. Check out the link here http://www.youtube.com/watch?v=-OQomrD0ODk