ALL OF SUDDEN STARTED TO PING NOT TAP OR KNOCK ITS A PINGING WHAT CAUSES THIS

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CAR RAN FINE FOR LONG TIME WITH THE CHECK ENGINE LIGHT ON HAD TO SMOG AND FOUND TINY VACUME LEAK THEN PASSED ALL OF THE CAR STARTED GETTING HOTTER TEMP IS FINE NOW AND SO NOW IT PINGS AND SMOKES BUT ONLY SOMETIMS