Engine Detonation and Preignition
Detonation is a problem caused by colliding flame fronts in an internal combustion engine. The spark plug creates only one of them, and the other is undesired. When the air/fuel charge ignites out of time and elsewhere in the chamber, a separate and undesired flame front ignites after the spark plug fires. These competing fronts create a loud knock when they collide. It can result in severe engine damage like bent rods and damaged pistons.
There is a difference between detonation and preignition. Mostly regarding when the flame front started and how it propagates through the chamber. Preignition is when a flame ignites in the combustion chamber before the spark plug fires and reveals itself as a slight knocking sound heard during acceleration. Knock occurs when the peak of the ignition process no longer occurs at the optimal moment in the engine's timing.
Most engines have a knock sensor that retards the engine's timing to prevent this harmful condition. This condition raises cylinder pressures. The competing fronts collide creating this audible knock that resonates down the piston, through the bearings, and onto the crankshaft. Ouch!
There are several causes for these conditions. Most have to do with temperature and timing. Other factors to think about include, has the engine been modified or have there been any changes to the ignition timing recently. Has the customer used a different fuel? Fuel octane rating has a lot to do with its ability to resist early or preignition. Check for TSB's in case of any inherent issues with the make and model you are diagnosing.