Common sense dictates that, if the unit contains 2 identical drivers, and only one rattles, there is a problem with the rattling one. As was mentioned, check the tightness of the screws on both drivers (always a good idea on subs!). You may also wish to carefully open the sub to make sure nothing has come loose that the cone might be hitting. If there is no obvious reason for the problem and you purchased the sub new, it should be repaired (or they may send you a new driver to install yourself) under warranty.
My credentials: I have been an audio manufacturer for over ten years, an electronic technician for over 30 years, and an audiophile even longer. (IMHO, anybody offering advice beyond what components they like or dislike should indicate their technical background).
My credentials: I have been an audio manufacturer for over ten years, an electronic technician for over 30 years, and an audiophile even longer. (IMHO, anybody offering advice beyond what components they like or dislike should indicate their technical background).