I have this also occasionally. Like you, it's only on some startups, never during idle or when taking off from a stop light, etc. It figured it was the turbo seals or just the atrocious vapor recovery system.

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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