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Well the valves are really not making the noise in the engine. What is happening is the lifters are not holding pressure and bleed down when the engine is off. After a few minutes of running they pump up. One thing you can try is run a thicker oil in your vehicle. Is this a bad sign, well over time it will get louder and your vehicle will start to run rough. I would have it looked at in time and ensure you dont damage anything internally. Let me know if you have any other questions.