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.
A) A knock sensor is a specially designed listening device for car engines. It listens for engine knock, or detonation (pinging). Detonation can be very bad for an engine and is the result of the fuel/air mixture exploding too quickly instead of burning evenly and rapidly.