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Mine rattles when I have a can or bottle in it... but it's the can or bottle rattling against the side of the cupholder, not the cupholder. I'd guess it's may be the same thing you're hearing.
Try this - get some black adhesive-back pads like these (click here) and put them around the rim of one of the cupholders where the rim of your favorite drink cup hits. See if it helps.