i have a 2008 BMW 335i and it makes this noise on start up. this isn't my car, i found this on youtube, but mine sounds exactly the same:http://www.youtube.com/watch?v=J-Bd4_7V_zc&feature=related i went to the dealer and they replaced the left muffler and it fixed it for about a month. now its doing it again. if all it is, is a noise when the car starts up then i won't replace it again, or does this affect the car in any other way other than that rattle noise?