nissan altima engine rattling noise

My car engine has been rattling for a while now. You can hear it when the car is at a stop. Also, you can hear it if I turn on the A/C or the radio. The car is a 2006 Nissan Altima 4 Cylinders A 2.5L and has over 100k mi on it. My best guess would be the alternator, but I'm not sure. Any thoughts?