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.
Protection. Knock sensors also protect car parts from the damaging effects of engine knock. When a knock sensor is bad, it can't regulate the engine properly. The car vibrates and it sounds like there is something banging up against the car's cylinder. The knocking action can destroy the pistons, rods, valves, gaskets and plugs.