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.
Knocking in diesel engine. Its also termed as detonation. Knocking is just a common term that says that there is some thing wrong in the engine , based on uncontrolled combustion process. Its called as uncontrolled combustion as its not occurring in Ideal time. Short answer.