I had the same problem. The power steering fluid was low and on hard corners, the pump was sucking in air. After a bit of driving, the air would purge out of the system and the sound would go away.
Where the power steering fluid went? I have no clue. I've never had to touch the power steering fluid on any car.
2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R