What might a grinding noise during engine start-up signal?

A grinding noise during engine start-up is often indicative of a faulty starter motor. When the starter motor is compromised, it may fail to engage properly with the flywheel. This misalignment can cause the starter gear to make contact with the flywheel inappropriately, resulting in a grinding sound.

In contrast, low battery voltage typically results in a clicking noise rather than grinding. Dirty air filters primarily affect engine performance and fuel efficiency, not the starting process directly. Additionally, fuel pump failure usually leads to engine cranking issues or no-start scenarios, rather than a grinding noise during startup. Therefore, the presence of a grinding noise strongly points toward a problem with the starter motor, confirming this as the correct answer.