I assume you can't tell where the rattle is coming from when you put your ear near the cabinet? I would say go down to your local medical supply store and pickup a stethoscope. See if you can tell what area the rattle is coming from. At the end of the day you may just have to open up the cabinet once you find the general vicinity of the noise and tighten down anything that may be suspect.

Two common causes of speaker rattle:

1. Damping material inside the box has shifted and is  making contact with the woofer cone.  
2. Foreign particles in the magnetic gap.    Metal shavings or ceramic magnet chips have migrated to the voice coil gap, then scrape against the voice coil.  

Thomas

Thomas Barefoot, President/CTO, Barefoot Sound (manufacturer)

In my then brand new Focal 706's. I was only in mid range and only in a few specific harmonics.  But it was very annoying.  Turned out the lead wire to the driver had come untaped from the driver basket and was buzzing against the cone at specific pitches and harmonics.  Definitive drivers only cost a few bucks a piece (GASP, GASP I say) and you could probably source one from the factory or eBay for less than you could spend having the driver dismantled, cleaned, and regapped - assuming that were the issue.

My guess is that it's not an issue with the driver, but a small construction issue with the cabinet.  Not sure about new ones, but Definitives used to be made out of Medite and MDF, which were pretty sturdy from a damping stand point but didn't always take to drops or moves very well.

Good luck.

I removed the packing from my Boston Accoustics and found some strands of cotton that I pulled out with needle nose pliers, and my lovely sound is back.  I am so grateful. Thank you.