Let me start off by saying i am not a mechanic. I have an 04 (new body style) screw lariat 4x4 with the 5.4. When i first start the truck after it has been siting for a while it has a rattling sound coming from under the hood for just a few seconds then goes away. A friend of mine said it sounds like the timing chain has some slack in it and is slapping against the timing chain cover until it tightens up after a few seconds. I have been reading a lot about people having problems with the cam phasers. Not sure if that is what i am hearing or what. Please help.

 

If I had to guess you have a tensioner gasket blown out. Oil galley will drain down when its off and the chain will slack off, then tighten when you start it and build oil pressure. A phaser will usually tick hot idle.

It might be worth spinning on a new oil filter first, I have seen a few come apart and cause weird oil pressure problems and actually plug up the galleys in the heads. I would put my money on the tensioner though.