I have a 1998 328is and have a rattling noise under the hood. It has something to do with the a/c. When I turn on the a/c it makes a rattling noise, but the a/c works great. It is loud enough to hear it at idle and the hood closed. I asked the place where I bought it and they said it is common for the a/c to make that noise and they said if I replaced it that it would start making the noise again in about 4 months. Any clues on what it might be.