I can't really give any diagnosis with out actually having it in front of me.
All I can say is-
Some subs make a "rattle" noise when the voice coil bottoms out against the backplate.
Tinsel leads can bounce off the cone during heavy excursion and create a "rattling" noises.
A magnet not properly secured to the woofer basket can cause rattles.
Im stumped-lots of possibilitys.
I would definitly call the manufacturure and describe this problem, you may have a "factory blem" which happens to even the best of company's.