Noise from the breaks comes from the so-called stick-slip-effect. The stick friction is higher than the slip friction. If there is some play between the pad and its seat in the caliper, the pad sticks at the disk until it hits the seat, then it "breaks loose" from the disk, starts to slip and will be pushed back because it is a little bit elastic. Then it sticks again and the whole thing starts from the beginning. The normal play for the pad is in the some 1/10 mm range so you can imagine the frequency is quite high if you break at 30 mph. This is what you hear.
How to avoid it? Small play for the pad or some damping, i.e. copper paste between the caliper and the pads. Also a different pad or disk material can help to get the resonance frequency out of reach.