Both my rim brakes --- front and back --- have started to make a periodic noise when I engage them. They will be silent for a little bit, then gradually start to make noise. As I brake, the noise will increase and eventually culminate in a very loud noise, and then be quiet again to restart the cycle. This is definitely independent of speed or how much I've been riding that day. The cycle seems to repeat itself every 1-3 days.

The noise is best described as finger nails on a chalkboard. It is definitely not a squeal.

Why does this happen? I suspect my pads are getting worn, but they are not past the indicator lines yet. The bike is a little over a year old and has gotten "medium" use.