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 spark-ignition internal combustion engines, knocking occurs when combustion of the air/fuel mixture in the cylinder. starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture combust. outside the envelope of the normal combustion process.