Pinging/Pinking (called both by various bogans) sounds like you have a light rattle in your engine under load or at increased rpm. Similar to a small hammer tapping quite quickly inside your engine.
The noise is most commonly heard in standard cars when people try to drive out of a corner in 3rd or 4th gear at 500rpm, the rattle (pinging) sounds like the engine is a diesel.
It's a very distinctive sound once you know what you are listening for. It should only be evident in your engine under load. If you have an engine probelm across your entire rpm range including idle then it's a problem other than pinging.
For the actual details of what causes pinging and how it occurs look it up on google.
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