Rapid Clicking Noise at pre-Start

I have a 2006 F-350 with the Triton 5.4L 3V and when I turn the key to the ON position, I get this rapid clicking from the engine bay for a couple of seconds. If I try to start the truck while the clicking is happening, it turns over for an extra 1-2 seconds before starting. If I wait until it's done, it starts and runs like normal. Just changed the baterry and thought it might be the starter, but this occurs before I actually try to start the engine.