1-2 Second Engine Knock Upon Start - 3.5 EcoBoost

[Josephm]

Reminds me of a common issue found on the Mazda MZR, which was due to a faulty lock pin in the variable valve timing actuator - number one symptom is a loud ticking/clacking for only the first couple of seconds at start-up. And it was due to slack in the timing chain, and the chain slapping against the underside of the valve cover very briefly.

VVT issues? Is that what you speak of? 

Always began on cars with around 50-75K miles on the odo.

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[painterpatt]

Mine does it. Motorcraft filter, seems to have gotten more frequent now that I'm in Florida. I had my wife start it one morning while I had my head down near motor. Sounds like its coming from throttle body area. More of a rattle than a knock.