I have 2 12s and whenever I press down on my trunk the rattling stops, but of course I can't hold it down when I'm in the car, what's that problem

 

It's not heavy or absorbant enough to "absorb the vibration", so when you push down on it the vibration levels out and is absorbed by the weight applied and the distribution of it along the trunk where it mates back to the car.

 

The problem with it is if you are not familiar with it there is a good chance it will expand to much. I know some people who made that mistake. It destroyed their trunk and imploded panels.

 

That's why you have to apply just a little bit at the time. I did it this way on my previous car and it worked!!!!