A friend of mine has a 99 540i and it has one hell of a weird sound. It sounds like there is a belt slipping. We have determend that there is massive crank case pressure or vaccuum problem if you pull the dipstick the sound goes away. If you lay under the car while it is running you can hear and feel the air coming out of the back of the oil pan. Which leads me to believe it is either the pcv system or the vaccuum pump. Does this sound logical or has anyone had this problem. Any help is appreciated. Thanks