[ I am not familiar with sxs guns.]
[I took the 3 screws on the bottom of the receiver out but it doesn't seem to release the trigger assembly.]
That's exactly what NOT to do, and most likely the source of the rattle, when someone else before you tried the same thing - unless the safety spring's gone sprong.
Unless some has a lot of experience with SxS shotguns, the most that should be done is to pull the buttstock for a clean/flush/oil.
Please take it to a gunsmith.
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