2002 4.6 Liter Engine Knock

The 2002 Explorer Limited that I just purchased developed what sounds like a low octane fuel knock. I would say this might be the case except that I filled it up the first time with Sunoco Ultra 94 and the tank was almost empty when I bought it so I doubt it's related to low octane. It almost acts and behaves as if the ignition timing isn't advancing at all as it knocks even just off idle. It does go away when not under any load at all. It also starts right up and idles fine. I would say some sort of sensor, but there is no check engine light on at all. I even tried clearing the computer by pulling the positive battery cable and that did nothing at all. Any suggestions before I bring it back to the dealer I bought it from as I did purchase an extended warranty.