As someone suggested up thread, disconnect the load side wires from the GFCI and see if it still trips. If not, check all of your outlets to see which ones lost power when you disconnected that wiring including ones outdoors. Electricians will often run wires in ways that don't always make sense once the drywall is up. I have personally seen porch outlets on the front of a house not 3 feet from an outlet on an interior wall connected to a GFCI on the deck at the rear of the house to save the cost of a second GFCI in front. You may be just missing an outlet or load that isn't obvious.