If ur into foaming get this stuff. Its way nicer to use than Great Stuff and is black.
You can find a gun and can kit at Amazon.
Along with reducing panel resonance, filling cavaties with foam will take that "seashell" effect out of the dead empty spaces and also helps seal things off. Keep bad sound out and good sound in.
I will share the vital areas I find as I go but pay close attention not to foam your wire harnesses away....not that the foam is hard to remove but just tedious.