I agree it looks like a housing endcap. what it is doing there is anyones guess. anything odd going on in the adjusting barrel?
I do like the rattle idea though. kind of funny as when I first starting riding seriously (you know nice bike training/group rides) one guy I rode with had a DA index bike. you could always tell when he was shifting because of how loud the
click was coming from the carbon tubes.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk