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.
Owner's manual: Unleaded premium gasoline, pump octane number 91 or higher. Use of lower octane gasoline can cause occasional metallic knocking noise in the engine and will result in decreased engine performance. Use of gasoline with a pump octane less than 87 can lead to engine damage.. 2013 Chevrolet Volt.