I went straight to the source...
Most rattles are caused by imperfections. Imperfections are caused by unwanted movement, that's why the cars don't rattle from the factory, you guys know what I mean, I'm sure.
Anyway, ITW foamseal makes a special foam that you inject into your frame and it makes the car more solid, and less chassis flex (so good that it's just like putting on front and rear upper and lower strut braces, or any brace of the like. It even makes the car handle more aggressively. It got rid of MOST of my rattles, but it's hard to say what it would do to yours, at the last point of driving the car, I still had no interior, and screws that were just sitting in their holes as well as on the floor rattled and moved. There was still a rattle in my dash that I still have not figured it out, but I have 3 dashes, so I will continue to change them out until I get the quiet one.