Subwoofer Rattle

I have a B&W ASW850 subwoofer that is rattling in the 20-40Hz range (tested with a signal generator). Although I have never consciously over driven the unit, I assume this just means the voice coil is shot and I need to replace the speaker? How easy is this to do?

Thanks for your help.

It's the speaker that is faulty. I took it out the cabinet and hooked it up to another amplifier (Quad 606 MkII) and signal generator. Sure enough, it started rattling badly below 52Hz. Here goes ~$700 to order a replacement speaker (p/n ZZ14102) from B&W NA.
Turns out the spider had come un-glued from part of the cone, the voice coil is undamaged. In the end I purchased a new subwoofer from B&W ($700) and everything works fine now. Allen Speaker Reconing in Houston are repairing the original speaker for me now and I will either keep it as a spare or sell it on.
I had the same sub and it rattled like heck. I discovered others with the same problem on an Internet forum. Apparently it was the main driver rattling against some bracing.
I replaced it with a Paradigm sub. No problems with that.