This Threads Sucks. Big Sucker and stock cover, no extra noise, no degradation of performance.

 

Did you ever figure out what was causing this noise?

 

It's the air cleaner. It's how much air goes in at a certain throttle position. Nothing to worry about. I just give it a little more gas and it doesn't do it.

 

Mine did the exact same thing. The way I fixed it was by removing air cleaner to expose throttle blade. Then I stuffed a moist rag in the throttle body behind the blade with just a tiny bit hanging out to grab for removing. MAKE SURE ITS LINT FREE! Then I gently sanded the Face and lip of the brass plate with 1500 grit until that lip is smooth. Remove the rag carefully and clean out the channel of the TB with carb cleaner and a rag. Worked like a charm. The plate has a leading edge lip from machining and with more air flowing past will sometimes whistle or whirl. This fixed mine.
Good luck.