Engine knock can be caused by an incorrect air/fuel mixture within one or more cylinders. The knocking sound is the fuel burning unevenly and creating shock waves at the wrong times. The waves can damage the cylinder wall as well as the piston.
Some vehicles require fuel with octane ratings higher than the normal 87. When a lower-grade fuel is used, engine knocking may be the result. Octane booster can be added to make up for low octane fuel, but it is better to use the higher-octane fuel. Using fuel with a higher rating than recommended by the manufacturer does not have any adverse effects.
Another reason for engine knock may be carbon deposits on the cylinder walls. All fuels for sale in the United States must have levels of detergent to clean out carbon. However, sometimes this is not enough to halt the formation of deposits. The deposits cause compression in the cylinder to increase, while decreasing its volume. Adding a secondary detergent may help alleviate this.
Incorrect spark plugs can also cause engine knock. All manufacturers recommend certain spark plugs for their vehicles, with some listed alternatives. If changing fuel grade or using additives does not solve engine knock, then the spark plugs may be the culprit.