I believe I know that sound. I hear sometimes when I start my car and asked the dealer about it once. I was told it has something to do with the fuel pump priming up or something to that effect. SA said it's not anything to worry about but we'll see. I though it was from under the hood but it was from the backend but the sound seems to echo under the car.
2008 650 Convertible- 6 Speed manual- Sport package- HUD- Lane Departure- Comfort Access- Merino Leather- HD Radio- SAT