If anyone has a rattle/vibration noise under the hood near the firewall, I found the culprit. I read on a previous thread that a vibration noise can come from the heater core piping behind the glovebox. I tried that, but not gone 100%. I was putting windshield wash fluid in yesterday, and took a look around before shutting the hood. The plastic cover over the top of the engine was vibrating in the back side on the hood when closed. I put some black foam weatherstrip type material along the foot and a half long top edge, and it is gone!!!!. 1300 miles on my 2012 Sahara, and no abnormal noise under the hood. Knock on wood.