vibration from spoiler

i have a 12" subwoofer in my trunk of my 03 zts sedan and when the sub is going, i can hear a rather annoying vibration from my rear spoiler if i'm
walking up to it from outside. anyone have any ideas of if this thing is hollow or not so i can fill it with monofoam or something?
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They are hollow. You can also put rubber grommets between the license plate and car to take care of that rattling noise.
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