MOST LIKELY cause is the coolant expansion tank is cracked and is leaking. This is a very common failure on your car. A lot of times it comes from overfilling the expansion tank, then after it pressurizes, it cracks around one of the hose fittings.

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2011 328i Convertible : Mineral White; Oyster/Black Dakota Leather; Premium, Sport, Value; Sport Wheel w/paddles; Heated Front Seats; Satellite radio; HK Sound
2012 528i Sedan : Titanium Silver; Oyster/Black Dakota Leather; Anthracite Wood; Comfort Access; Side & Top View Cameras; Cold Weather, Premium; Technology;
2004 330ci : Titanium Silver; Gray Leather; Premium, Sport; Fold Down Rear Seats; Xenon Headlights; Navigation System; HK Sound
2001 X5, 2009 528i (retired)