Buy yourself a speaker kit of Dynamat from Halfords or an ICE dealer. Take it to the dealer and ask them to take the sub out, stick a sheet of it in the enclosure, and use the other bit to cut up and stick on the back of the panel. Not vibrations and 10 times better sound. Or do it yourself.
Almost all cars with 'subs' in them rattle. No car is built to take 40Hz bass frequency at high volume, not rattle and make money.