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.
There are a few reasons engine knock is so confusing. First off, the name simply sounds intimidating -- knocks are hard and often imprecise sounds, and if they're being produced by a lot of moving metal parts, that's probably not good. Engine knock is also known by other terms, which only adds to the anxiety. If you've heard the term detonation, it's pretty much the same thing as engine knock. If you've listened to your engine for a while and you're convinced the knock sounds more like pinging, you'd also be correct.