I bought a dynamat xtreme bulk pack to try to tackle my severe trunk rattle. I only used about 4 sheets out of the 9. I dynamat'd 90% of my trunk lid, as well as the far back wall of my trunk (closest to rear bumper). The rattling went from a 9/10 to a 7/10. The SPL inside the car improved probably about 150%. I bet if I put another layer on the trunk lid it would improve SPL even more. My car is a 2D coupe, if you have an SUV you probably won't notice as much of an improvement in SPL as you do in a car with a closed trunk. As for stopping the rattling, try finding some high strength expanding foam spray. This will not improve SPL at all but will expand inside the rattling area and physically hold the pieces from moving. Dynamat is more for improving SPL, expanding foam is probably the best for stopping rattle.