[Rooster's 67]

make sure the drums after turning are smooth and don't show "chatter" or "threading". They can be caused by improper machining technique. Chatter will look like a diagonal pattern most times and threading is cause by too fast a feed speed during a cut causing what will look like threads of a bolt(all be it very fine) both can cause funky noises/vibrations during braking.
Like Longhair said if the spring is missing from the outside circumference of the drum you will very likely get harmonic noises.