Pouring water in the system is a good idea. It'll show you where the leak is. You could also grab the water pump and I'll bet it has play in the bearing.

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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