Whine or Whistle?
The 750's have an odd reed valve in the intake that, at just the right speeds, makes this, almost rattling high-pitched sound. They don't do it new either. Takes a few hundred to first start and a few thousand to get loud. I've got almost 6k on my 08 now and I've gotten used to it now that I figured out what it was. There's a way to shut it up with a BB or something, but now that I know what the noise is, it doesn't bother me. (I have a Jeep, want to talk about a vehicle that makes some odd noises... LOL!)
But... How to describe that whistle.... hmmm... like maybe a shoe on a basketball court type sound, really fast (engine pulses), but yet sounds almost like a rattling piece of metal. It's an odd one to be sure. Sometimes the exhaust is a bit lose which seems to make that type of sound also but worse, snug those up to 18.5 foot pounds I think it is (which really isn't all that tight) and maybe check the intake to make sure it's seated.
If that's not it... BUT it does go down with the engine speed... Honestly I'm not sure other than a gear inside the engine...
By now, you should have had your valve adjustment done. Make sure the air-box didn't come lose now. It does from the vibrations now and again. I put it on my 'check monthly' list along with the exhaust bolts.
Just a thought.