If it is in any way repeatable, I suggest taking it in. Here are my wild-ass guesses in order of increasing probability:
1 - Unlikely - Wastgate rattle. Would sound very metallic and possibly louder when the engine is cold.
2 - More likely - Something stuck under the car like you guessed
3 - Most likely - Foreign debris has gotten under or behind the cowl leading to the blower. BMW has a service bulletin on this and there's some kind of foam seal they replace... it's been a while since I heard that... best to ask.