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.

Hmmm...if the whole thing needs to be replaced...sometimes they do that...the voice coil, magnet, etc...that probably is a good portion of the sub. I think it is pretty straight forward...just might be expensive. i am NO i admit i could well be wrong, but i have replaced entire cones, a tiny inexpensive one can be 20-50 expensive one can run a few hundred.
Sounds to me as if you will have to replace the drive unit. B&W has them in stock here:
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.
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.
Rattle could be one of the following:
1) Vibration/rattle of nearby furniture, wall hangings, wall or floor
2) Vibration/rattle of a loose speaker grill
3) Driver could be striking/rattling the speaker grill with long excursion if over driven (reduce the power if this is the case)
4) Blown driver
Check all possibilities before replacing the driver.
Good luck!