Quote:
Never heard it called that befor....
A US term??

also known as pink or pinking

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 ignites the mixture. 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 spontaoonious ignitiion 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

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.

Check the site below;it is a good start to understanding the reasons for certain sounds your engine may make.

http://en.wikipedia.org/wiki/Engine_knocking

I read that you hear an engine ping when trying to set the timming... What is known as a ping?