is imo without seeing the car caused by the axle pipe bumping up against the piece of steel which in a rear end impact helps to keep that section from according into the rear seats. get down on your side in back of the rear wheel exhaust side and follow the axle pipe as it makes its way to the rear most muffler. down around that area you will likely find the axle pipe is very close to this steel plate.

if this is so you have two working choices.
1. loosen the clamps and rear work the pipe to allow for more room

or 2. get a piece of rubber......an old large 240 round rubber muffler hanger works PERFECTLY and cut it, slip it around the axle pipe and where it is closest to the frame section and then use a couple of large plastic cable ties to secure it in place. this is not an ideal fix but it will stop the banging and be quite durable as there isn't enough heat at that point to ruin the rubber.