I'm better the 'whistle' you heard was the turbo spooling up. Common sound for turbo diesels in my experience (I've previously owned two different turbo diesel trucks).

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Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual