I noticed the same thing. Some of it could be the rear seats muffling them too.
This guy has an EXCELLENT website with instructions (& pictures) on replacing the stereo & speakers:
I'm worried about rattles too. I had several in my last car after replacing the speakers that I could never fix (part of the reason for the trade).
For now the stock speakers sound OK given that I have spent quite a bit of time messing with the controls. It doesn't sound too bad with the treble all the way up, the bass about three quarters, and the midrange about a third of the way up. I'm not sure if the improvement would be worth $200 - $300 to replace the speakers, and I'm also not ready to replace the head unit.
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