Hi again,
It is more of a feeling than a noise to be honest... Last time it happened, I tried to listen for it bit realised I could only hear wind noise in my helmet.
Also, I studied it more and can feel it in the handlebars like I already said, but also feel it in the pegs and the seat. It feels like the engine just starts making more friction at 6k rpm. Also, I definitely found that speed doesn't matter, it is only the revs, because I can feel it in several gears at those revs. 5.5k and down, 6.5k and up feel smooth all the way.
Also, if I am say at 7k and I lift off the throttle slightly so that the revs drop to 6k I think it can feel more pronounced than if I rise up to 6k. I found that very weird.
I'm an acoustic engineer and I had a weird thought that maybe it is something to do with the air intake. It is very possible that the engine draws air at such a speed (at 6k revs) that it corresponds to the resonant frequency of the intake pipe and makes a really weird sensation. Something like a Helmholtz resonance (like when you blow across a half empty bottle and it sounds like a lighthouse, then you can feel the bottle vibrate)... It feels like that sort of thing but because I can't hear anything for the wind noise in the helmet, I can't really confirm. An acoustic resonance would correspond to the gradual build up and tail off at lower and higher revs than 6k... And also the preciseness of the problem.
Also at lower speeds when I can hear more through the helmet, there is a strong sucking sound from the air intakes so I will check if perhaps something is obstructing it in some way, or just normal behavior.
I've checked everything mechanical I can on the bike:
-Wheel bearings (Front and back)
-Chain tension and alignment
-Engine oil level
-Brakes don't stick at all (as far as I can tell from checking the pads and moving the wheels while on the center stand)
There is a rattle in the fairing exactly as described by the headlight but that occurs at other revs and I just pressed the fairing as suggested by the left headlight and it went away... So I'm not worried about that.
I haven't noticed any lag in power because of this though... Just the feeling.. So I dont think that anything is being degraded or broken.
Maybe I'm just too particular, and I guess I could have written a book about these sorts of things with my old XT600E hahaha :-)
Once again I appreciate your help and suggestions guys! Many thanks :-)
James