I've never noticed this on mine. My drivers door speaker did start to sound muffled/distorted so I had that replaced under warranty last week and now its all fine again.
It's the rear on mine - I am guessing that it isn't secured properly because I can stop the buzz by holding the speaker surround (not easy whilst driving). Just don't know how to remove it safely so that I can have a go at resecuring it. What about a dab of superglue?
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