it doesnt sound like the speaker itself that is rattling.the best way to find it is to start with the speaker.pick it up and hold it while its working.does the sound go away?if so look for something near the sub that is vibrating in sympathy with it.if you still hear the sound and its coming from the sub its something loose in the enclosure.i would return it if this is the case. if its on the floor it could even be the floor making the noise.