Are these sealed subs? Do you have model numbers?
Here's my experience with sealed subs:
I had a rattle a year or so ago. This sub had a sealed amp section within the sub's main enclosure, in an effort to seal off the plate amp's electronics from the back pressure of the driver. After taking everything apart, I noticed that the hot glue applied to the edges where the amp box met the sub enclosure was brittle and gone in some places. I picked out all the remaining hot glue around the entirety of the enclosure and used a nice heavy bead of caulk around every edge on the interior of the thing. Wow, what a difference! It was like night and day.
I bought another matching sub off ebay and the first things I did was to tear it all down and recaulk it. These are 12 year old subs, and they sound brand new. Keep in mind, if you caulk the interior seams, let the caulk cure for at least 24-48 hours before reinstalling your driver. The offgassing can eat away at some driver surrounds.
Also, when you reinstall the driver, make sure to scrap off all the dried crap adhesive off the driver flange. I reinstall using a nice bead of "rope caulk" weatherstripping on the driver. A lot easier to remove if you ever need to get the driver out.
Since then, I had a plate amp die on one of them, so I replaced both amps with matching new ones, and moved them outboard in their own enclosure. (Even less risk of something rattling.)
For 12 year old subs, with original drivers I can play X-Men First Class, Inception, etc. at near reference levels, (-5 or so on the AVR), with absolutely NO rattles. (I'm also using AntiMode 8033). If you have Audyssey or the like, you should be okay.
Hope this helps.