so i just noticed this noise a couple of days ago. I had my music off and turned the engine over. For about 30 seconds after being started up, i hear a rumbling, almost grinding noise from the rear of the car, you can say coming form the trunk/spare tire area. It only rumbles for 30 seconds and then thats it. quiet afterwards. anyone else?

 

May be your fuel pump. If you turn the key to on but don't start the engine you should hear the pump "prime" the system. That should be your sound. Otherwise, I don't know what else at the rear of the car would make that kind of sound during start-up.