So I just changed oil after running seafoam through crank and fuel, bike's much smoother.
I noticed something odd though. Right at 6000rpm (which translates to 100 times a second), there's something in the bike that vibrates. It sounds almost like an electronic beep... if anyone's wired up those tiny beeping circuits as kids then they would know. The first time I got stressed out 'cause I thought something beeped when I revved...
I call it resonance since it's exactly at ~6k rpm. I have a hard time getting it to sound since the rpm has to be so exact, but its there distinctively every time I hit the spot.

Is the F4i horn a 100Hz tone? In that case the horn's diaphragm could be vibrating with the engine at 6k... anyone?
(no I dont think it's CCT - that's a different sound that I hear&feel faintly in a wider range of rpm)

 

Does it make noise when you are in nuetral at a stop, and just reving it up top 6000 in the garage. Does it also make the same noise outside under the same conditions. If not I would say something in garage makin noise and not the bike. Get a buddy with damn good ears, good luck.

 

I had stuff rattling in my upper fairing at about 6500rpm, but it was a definate rattle. turns out I had a busted mount.

like ruftags said, you need to start narrowing it down. does it do it in or out of the garage, moving or stationary, or all of the above?

 

It makes the sound only while moving... I've noticed it in 1st gear so far. It definitely sounds like the horn, in fact if I'm close to a car I fear it'll be mistaken for a beep.
I revved to 6k rpm when stationary... nothing. Then I put it into 1st gear and revved again - nothing again. Only when I was moving. Next time I'll pull the clutch in while moving and rev... if that'll mean anything.

Could it be that while moving it vibrates more, so the beep sounds, but not when it's stationary...?

 

unplug the horn, possibly even remove it just to be sure that it is or is not making the noise.

unplug it first, then test. if it doesnt do it, then you have a short somewhere.
if it still does it, then remove the horn, test,
if it still does it, then its not the horn----you just got something rattling