Chevrolet Repair Questions? Ask a Chevy Mechanic for Answers ASAP
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P0402 means the onboard computer has detected excessive flow from the EGR valve - which could also account for the tapping noise. There are tests that can be run to confirm whether this issue is caused by the valve itself or by the onboard computer - problem is you'd need a high-dollar scan tool to run those tests.
So, taking into consideration that the EGR has been there for over 300K, I'd gamble that's where the problem is. You have at least two options on what to do to fix this issue; you could replace the bad valve with a new one, or you install a delete kit with an intake elbow (click here to see both options on ebay)
Please let me know if you have any other questions.
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OK. Please let me know how you make out.