Oil loss Mini Cooper S

Mini Cooper S I drive a Mini Cooper S from the year 2007. I love my sweet but I have to refill every 1000 km 1 to 1.5 liters of oil. I have been to the BMW dealer so many times and they tell me really, yes the problem is known but you don’t know where it comes from!!!!! So it’s my problem and in the worst case the engine gets broken at some point. I hear a rattle and then know oh oh now quickly check oil level and then it’s almost empty again. who can help me? Does that know what it might be or does it have the same problem? Greetings mini616