Hi everyone:
For the last few months, every once in a while when I turned off the engine, there was a real loud "whoosh" suction/vacuum sound under the hood. Did anyone experience this sound? I know that it is not normal but I cannot tell where or what broken part/s the sound came from. I cannot recreate the problem, it just comes and goes unexpectly. Help please?

2005 X5 3L, 119,xxx miles.

Thanks!