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.

What do you mean by rattling? Have you tried to push on the cone to see if the voice coil is actually touching the magnet? If you have never driven it hard, it is possible that the problem is in the built-in amp and not the voice coil.
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.