The traditional description is "pre-ignition" that is the gas-air mixture is ignited before top dead center causing the position to knock. Diesels count on this to function, hence the noise. Usually, a hot spot caused by carbon deposits is the cause of the knock. A mechanical knock can be caused by a failing connecting rod as well. Have you changed your plugs?

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2013 X5 3.5i M Sport
2004 M3 cabriolet SMG