actually, my filled-blanks crackle too. most of the time. it is a slightly different crackle though, a bit less pronounced, and sometimes the prefilled's give a big POP, and that usually never happens with my filled blanks.
maybe because the prefilled's have been sitting around longer, and the liquid inside may be slightly dryer, a little more gummy, increasing the viscosity of the liquid, which could increase the crackle/popping? Idunno