Notice that a few weeks ago, music was a bit high. The bass was deep. Got out to get my bag out of the trunk. Engine was running. I heard that rattling sound. Thought it was the exhaust, I turned off the car and listened. I then realized that every time the bass would pick up the heat shield under the car which is directly on the spare tire was the culprit. I tightened the bass box in the spare but problem still persists. Going to have the dealer take a look also.