First make sure it's not inside. There are two silver HVAC lines that run behind the dash - you can see them when you take off the glove box to the left in back. They run next to eachother and sometimes will rattle. Get a piece of heat proof rubber hose and separate them. They do get hot so don't touch them after driving.
2013 JKU Sport, billet, auto, 3.73, LSD, 315/75/16 Duratracs on MB 72, 10A springs, Bushwacker flatties
Sold - 2005 TJ, red, auto, 3.73, 33" Duratracs, 4" lift
Ice storm casualty in 2007 - 2003 TJ, green, auto, 3.73, 33" Duratracs, 4" lift