I'm betting you are over driving the sub, it isn't meant to pound with heavy bass just fill in the lower end, not that there still couldn't be something loose but I would guess what ever is rattling was brought about by being over driven inspite of the DSP. Perhaps the DSP program is a little too lenient allowing too much movement.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70