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.
In C.I. engines auto ignition is key to combustion, but if the fuel takes some time to vaporize & prepare for combustion then some fuel is accumulated in chamber when auto-ignition starts at end of compression stroke, which causes a sudden large amount of energy release and abrupt pressure rise, and causes knocking sound.