i have a 04 coupe that started making a loud rattling/vibrating sound from the exhaust(sounds like its coming from the muffler. everything is stock) the noise gets louder when the engine is cold and when you hit the gas..if i let it idle it will go away but if i tap the gas on N it will rev and the thump/rattle breaks in with the sound of the rev. can anyone please explain what the issue might be and what the fix is? or any self checks/test i can perhaps do to pin point the problem..thank you in advance