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.
Knock sensor is located on the engine cylinder block or cylinder head. It works on the principal of Piezoelectricity: when pressure or vibration is applied to piezoelectric material, it changes its shape and vice versa. Engine generates vibration when knocking occurs in the engine.