When I do a fast deceleration, I get a noise of something winding down.
Is it the electric fan being spun by the passing air, and is noticeable when I stop quickly

 

I get the same thing. Do you downshift as you slow? Becuase I have noticed it does it a lot when I click it into first with the clutch in.

MDFF

 

Yes, They were quick stops, not emergency stops, so I was downshifting quickly

 

Engine vacuum is extremely high on deceleration. Could it be a whistling sound from a vacuum leak?

 

Sound like there a bearing involved and maybe from the front to the bike, don't think it's directly connected to the drive path (drive chain)

 

the noise is comeing from the crank case vent tube . Most every bike ive owned has hist spit or sputerd. What u are is the back pressure of a higher rpm still comeing out. Mine has a check valve at the end of it. But please make sure its not in the drive line.[X(]