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Join Date: Aug 2007
Location: Northampton, MA
Posts: 956
Bikes: Iron Monkey: a junkyard steel 26" slick-tired city bike. Grey Fox: A Trek 7x00 frame, painted, with everything built, from spokes up. Jet Jaguar: A 92 Cannondale R900 frame, powder coated matte black with red and aluminum highlights.
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You know what? This sounds to me like a reflector or a big valve cap blinker or something. When the bike gets up to speed, centrifugal force pulls it out and it hits the chainstays.
Matt, if you can get the back wheel up (ideally in a stand) and spin the pedals, see if something on the wheel is deforming and hitting the frame.