weird rattle problem, need suggestions

[curtis96olds 10+ year member]

ok so today i put a couple layers of damplifier pro on the back wall of my truck, put my subs back in and the metal rattle is gone, but now i hear a different rattle...so i take the subs back out to see what it could be, and i narrowed it down to the little cabin vents that are in the back wall, the kind that flap open and they make one hell of a sound when subs are playing, but i didnt notice it before over the metal rattling. any suggestions on how to fix this? im sure some of you have ran into this before

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