I've got 2004 F-150 Lariat with a 5.4 engine. When I first turn on the engine when it's cold, it sounds like a diesel engine for three seconds--like a rattling sound that quiets down. What could it be? It runs fine after the initial rumble.
Unfortunately for you, that is almost always a testing slap. It means that the bearings are worn and that the pistons around are slapping back and forth. If it makes that much noise, have a mechanic listen to and diagnose the noise. Bring it to a mechanic and let it sit in his garage over night and the next day let him start listening. If that's the case, either decide to get another vehicle or put another engine in, because once they start rattling like that, there's serious damage inside them. I don't advise fixing them piece by piece and putting new bearings because you would have to do a crank over. And when you do the crank over, you have to pull the engine out and strip it down.
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