It's done this three times now. It only happens when the car has sat for at least a couple hours and the engine's cooled down. Upon start up there is a quick, but substantial, cloud of white smoke. After running for about 30 seconds the smoke is non-existent. There is nothing else out of the ordinary to accompany the smoke. It's at 150,000 miles and has never smoked in the past. Any ideas?

1999 e430