I have a 2008 Grand Prix gxp that keeps missing coolant. For the past three days the reservoir has been a half inch under the specified amount after I top it off. There are no leaks that I can find anywhere and the car runs just fine but the temp gauge will go about a centimeter over halfway when running. With the hood popped there is a sound in the rear engine bay that sounds like a vacuum leak but I cannot locate the origin of the noise. My concern is whether or not is is a cracked head or internal head gasket leak. Has anyone else had this problem?