2005 Ford Mustang GT engine/spark knock?

It seems to have a engine/spark knock at idle. I can take off and it drives fine. Oil pressure gauge is normal. Car just had a fresh oil change and air filter change. Oil is motorcraft 5W20. Car has 111k miles on it. If I turn the car off, and turn it back on the knock goes away. No check engine light is coming on. Any help will be appreciated

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