Started as I was pulling out of my driveway. I became louder with RPM. I thought it was the tranny but I put the car in Park and it still did. On my way to work I pulled into the local BMW dealer about 10 minutes after I left my house and a couple of guys (SA & Foreman) came out to see it, they thought it sounded kind of normal. Granted, the noise had subsided a bit but it was still there. When I got to work I turned the engine off and back on and then the noise completely disappeared. Then it hit me. The noise sounded very similar to what the car sounds like when you first start the car in a cold morning. Usually it dissipates within a few hundred yards. I think it's because most new cars are engineered to run the car differently when cold to warm up the cats quickly. Today it was 45 degrees out. This car has been started in 20 degree weather before, without exhibiting this noise. Anyway, if it does it again tomorrow morning I am taking it in.