Obviously, not being able to hear the rattle I can only guess, but turning the steering wheel is placing load on the engine. The steering system might not be related at all.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim