Had a strange one once, this is what I found.

There is a check valve located in the oil filter housing that may be a cause to your engine noise during startup. If you run the engine, such as driving to work, park then return to the vehicle in a short amount of time (2-3 hours) to leave and the engine has a unusual noise for a few moments, suspect this valve.

The chain tensioner tends to loose oil pressure because it bleeds past this vavle back to the pan.

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