My 1993 318is has the same problem. When I was still on warranty, I had it checked out many times and a representative from BMW said that this was completely normal. I knew the guys at the dealership and they said that it might be the throw-out bearing. They replaced it for me (under warranty of course) and the rattling was still there. The rattling seems to occur only when the car is running warm....the rep from BMW gave some lame explanation like it was the gears just vibrating against each other but who knows. So don't worry, you are not alone....hope you find a fix though!