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Pinging is a rattle in the engine on acceleration, most common on hot days, if the timing is too far advanced or if you are using low octane fuel. It means the air-fuel mix is igniting prematurely. It is very bad for the engine.
If you Hear It
Immediately reduce the throttle pressure (ease up on the accelerator pedal). If the pinging stops, good. Avoid pinging until you can have your engine tuned by a professional, or do it yourself. Generally it's easy to avoid pinging, just don't push down on the pedal so much.
IF pinging occurs often/random, have the ignition system inspected.
IF it only occurs on a hot day, consider lowering engine temperature with a 75.5c (170F) degree thermostat. Alternatively, retard timing just a tad (see Ignition_Timing#Fine-Tuning).
Overview
Engine spark knocking is is also known as detonation, in British English as 'pink' or 'pinking', and Ping/Pinging (US English) .
You May be using super-duper ultra high octane fuel, but if the combustion is initiated too soon, it will result in detonation.
Wikipedia article: Engine knockingᵂ
What does ping sound like? Like faint metallic rattling marbles in the engine as you push down on the accelerator pedal. Be sure to use a quiet exhaust system so you can hear detonation (or use detonation cansᴳ).
There are two causes of pinging in an engine:
- Pre-ignition. This is caused by a hot spot in the combustion chamber that ignites the fuel/air mixture before the spark plug fires. The Pinging sound is caused by the two wave fronts (spark plug explosion+hot spot explosion) meeting and the resulting expanding gases creating a frequency similar to the natural/resonate frequency of the metal around the combustion chamber creating a "pinging" sound.
- Detonation, this is the spontaneous ignition of the fuel air mixture before the spark plug ignites the mixture. It is caused by high combustion temps(high compression) and or too much combustion pressure (ignition advance + compression). Detonation is usually the one that puts holes in your pistons. The pinging sound is created the same way as above. It is from two pressure wave fronts meeting.