Definition of discourse marker noun from the Oxford Advanced Learner's Dictionary
When an engine starts knocking, one of the first things a mechanic will check is the oil pressure. A warning light may come on in the car, and adding sufficient oil will solve the problem. Sometimes, a knocking noise is caused by loose torque converter bolts or a loose flywheel.
Diesel knock is the clanking, rattling sound emitted from a running Diesel Engine. This noise is caused by the compression of air in the cylinders and the ignition of the fuel as it is injected into the cylinder. This is much the same as a gasoline engine suffering from pre-ignitionpre-ignition or spark knocking.