I have a 04 150 4.6 4x4 that has a rattle when it's under a load like pulling a camper or accelerating. It sounds like a hub trying to engage. I've replaced both hubs and solenoid. Done vacuum tests on the entire 4x4 system and nothing found. I've taken it to the Ford dealership. They ran tests on vacuum system, trim tests on the o2. Back pressure testa on cats. Nothing found. Does anyone have any ideas?