I wonder if your rattle might be the speaker mounting. Putting some rubber grommets or rubber gasket between the speaker and the mounting area could fix this problem.
If this isn't the source of the rattle, it might be the window and other components rattling from the speaker vibration.
I put the asphalt-based mat (Q-Mat I think was the name) on the floor area over the transmission hump from front to rear. This really reduced the drivetrain noise considerably.
