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.
Low quality fuel. One of the causes of engine knock is sub-par gasoline -- low quality and low octane fuel can cause a whole cluster of problems, such as increased combustion chamber temperatures and higher cylinder pressures.