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 this case, it is probably upper valve train or tappet noise caused by either low oil pressure, excessive valve clearance or a defective hydraulic valve lifter. The most common cause -- and easiest to remedy -- is that the engine is low on oil. Another answer is that there is some blockage in the system.