Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
That might be, unfortunately, a defective speaker.
Rattle in the bass, in a woofer, can simply mean 'overdriven' to above its capacities, or too much sound pumped through it.
On the other hand, check these things:
Anything on top of the woofer?
Inside it: is the speaker cone firmly mounted, or has it torn loose, come loose, is loose, anywhere?
is the speaker cone hitting the limiter when the rattle sounds? if so, you're simply overdriving it.
is any part with a screw mounting not tight? the speaker hardware being mounted loosely, a loose plate, a loose heat-sink, anything like that can contribute.
However... don't go inside it, if it is still within warranty. That would invalidate the warranty, if you opened the case. You might want to just make it a warranty claim to Klipsch because, ultimately, if you aren't overdriving it grossly, it shouldn't rattle. Getting it fixed or replaced for free under warranty terms would be a good way of dealing with it.
If this answers your question, please rate my answer. A five-star rating would be appreciated!
(If you need more info, or have any comments, use Reply To Expert to respond. And I will provide further advice.)