The rattle and crunch was a cool creepy effect suggesting there are spooky skeletons chasing you, emphasized with the tempo change and introduction of the synths/drum beat. Cool song that tells a story, though one thing I'll comment on is that I would've preferred it if the bass in the beginning had a slower, less intense attack (maybe even some delay)... after all, you're trying to establish an ominous, creepy ambience (mind you, the bass as it is works well in the more uptempo section).