I just bought a used BMW 318 IS with 118K miles. Everything is working great, even the air conditioner; however when I run the air conditioner, when I park and turn the car off approximately 3-4 minutes after the car is turned off I hear a high pitched (excuse the expression) fart-like noise that lasts for three or four seconds. Almost like the car is breathing a high pitch sigh of relief. Does anyone know what this is? I am guessing it has to to with the air conditioner since it only occurs after I run it.