This is the only explanation of the sound I'm hearing, and I'm stumped for a diagnosis. A metal ZZZZIP ZZZZZZZZIP ZZIP, mostly when in second, no drivetrain load, 27-33mph range. Sounds like a collapsing slinky amplified; bottom rear area or the drive train. Can't really say whether it's left or right. Much more predominant when the bike is cold; not fully warmed up and more noticeable down hills or when there is slack in the drive train. Anyone have a definitive diagnosis/fix for this noise? Is it the final drive flange bearing I keep reading about?