Thanks for all the feedback, everyone. I will first try adjusting the toe-in on the brakes. I don't think it is a matter of a worn headset since I just got the bike this year. I did replace the brake pads, and don't remember the chatter from when I first got the bike, so it sounds like that could well the problem (the pads being out of adjustment). Adjusting cantilever pads is such a pain in the butt.