Learn Spanish (Latin America): Revisar el engranaje si la máquina había hecho ruido. - Check the gear if the machine had made noise.

Using an infinitive like Revisar is a standard way to give impersonal instructions in Spanish. You will see this a lot in troubleshooting manuals, recipes, and signs. It tells the reader what action needs to be taken without addressing them directly as tú or usted.

Why use the past perfect había hecho here rather than a simple past tense?

This phrasing is typical for a troubleshooting guide. The past perfect había hecho (had made) emphasizes that the noise occurred in the background before the current situation, like right before the machine broke down or before you began this inspection step.

For "made noise," why use hecho ruido instead of the verb sonar?

Hacer ruido is the most natural way to describe an unwanted, clanking, or disruptive noise, which fits a malfunctioning machine perfectly. The verb sonar translates closer to "to sound" or "to ring," which implies a more deliberate or functional sound, like an alarm or a phone.

Does engranaje refer to a single gear or the whole gear system?

Engranaje technically refers to the entire gear mechanism or assembly working together. If you wanted to specify a single gear wheel, you might hear a mechanic use the term piñón or rueda dentada, though in casual speech, people often use engranaje for a single gear as well.