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 a diesel engine is different then knocking in a gas engine. Knocking in a gas engine is very destructive and is caused by two flame fronts colliding with each other. This is cause by certain conditions that can cause pre ignition of the air/fuel mixture separate from the spark.