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BMW has an issue with oil separators freezing in cold weather if the engine is not fully warmed up during driving. Common symptoms are a howling noise followed by oil leaks.
Crankcase breather valve used to purge the oil vapor from the engine block into the valve cover. This valve has 4 hoses connected to it, running to dipstick, valve cover, air intake.
You should also look into replacing your valve cover gasket when changing the separator.
