i had a giant, and then a nishiki, that rode great for a while, then produced the popcorn noise you spoke of... about 2 months after those two made that noise, the frame broke in exactly the same spot... the angled bar, right near the headstock (forgive me if thats not tech terminology). im not sure if the two events were related, but i would definately not wait on this problem, less you find yourself with no frame...