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.
Deposits on the cylinder walls. Yes, it's a vicious cycle -- the first cause of engine knock we discussed above can create bigger problems. Low quality fuel and pre-ignition forms the harmful deposits we mentioned, and once those deposits start, they tend to build.