i have had my x5 for 2 wks-it has been doing it from the start. i orginally thought it was a type of compartment decompression-due to how air tight the comcartpment is so you can shut the doors.(as is a mercedez/jag the windows drop so you can shut the doors). so i called the 800 # and they knew nothing about it. I called the service manager at the dealership and he said it is normal when you open the door it is the car waking up--and preparing itself for the start. I'm not sure, I buy that. but two night ago i heard a new sound, after an hours drive, parked in the garage-came in the house-after about 2 mins-went back into the garage-and heard humming or fan like sound (coming from the passanger rear wheel well) lasted about a min. Service manager said, going to sleeps sounds are also normal. And don't worry about it if unless it last longer than 30 min ????????????????????