Joined: Jul 2015
Posts: 7,476
Likes: 3,287
From: NW Oregon
Bikes: 1982 Trek 930R Custom, '91 Diamondback Ascent w/ XT, XTR updates, Fuji Team Pro CF road flyer, Specialized Sirrus Gravel Convert, '09 Comencal Meta 5.5 XC, '02 Marin MBX500, '84 Gitane Criterium bike
probably a too-smooth disc... typically known as "glazed" or "burnished"..... the pads grab/release repeatedly, setting up a rapid vibration we can hear...
lightly sand the disc braking surface, and the pads, too... i use 180 grit black emery paper....