I have a 99 F-150 with a 4.6 built at Romeo, MI in Nov. of 98. Recently I have heard a metalic tapping noise during cold starts that sounds almost like a diesel engine. It is not very loud and I can't tell from where in the engine it is coming from but it sounds like maybe the lower end. This noise usually lasts for a couple of minutes and goes away when the engine warms up. I have 16k miles on the truck and have changed the oil every 3k miles with Castrol GTX 5w/30 and Motorcraft FL-820s filter. I have babied this truck from day one so any engine problems aren't from hard driving. Any ideas what this could be? I have heard that this motor doesn't have the piston slap problem but I'm beginning to wonder.