Frozen up CCV system. Depending on what pops, it can shoot oil into the intake (your smoke) or pop the valve cover gaskets (the leak) or both. Expect puddles of oil in the intake.

It is usually not catastrophic unless you continue driving low on oil or the engine hydrolocks and breaks something.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles