I was listening to some music on low volume with the car off but key in the accessory position. After about 10 minutes I heard something like a small fan or something start to run in the rear of the car. I opened the trunk and it was a little louder. When I lifted the liner and exposed the spare tire is when the sound grew a little louder but I couldn't tell if it was coming from behind the side panel in the trunk. It couldn't be the fuel pump running right? I made sure the car was definitely just in accessory and not key turned on.