Originally Posted by
cyccommute
Toe-in on the brakes is probably your problem. Rim brakes are toed-in but discs aren't. The tails of the rim brake pads should be at least a few mm further away from the rim then the front (the toe). This keeps the brakes from lifting off the rim and chattering.
I have found that on bikes with sloppy brakes, a reverse toe, that is, the rear edges touching first, is the only thing that truly ameliorates the noise.