What is a common indicator that an air filter needs cleaning or replacement?

A decrease in fuel efficiency is a common indicator that an air filter may need cleaning or replacement. The air filter plays a crucial role in ensuring that clean air flows into the engine for combustion. When the filter becomes clogged with dirt and debris, it restricts airflow, which can lead to an improper air-fuel mixture. As a result, the engine may struggle to operate efficiently, leading to decreased fuel efficiency. This not only affects the overall performance of the vehicle but can also increase fuel consumption, prompting the driver to notice a drop in how far their vehicle can go on a certain amount of fuel.

The other options can be related to various other issues within the vehicle and are not as directly connected to the condition of the air filter. Unusual noises from the engine can indicate a range of mechanical problems, the engine light turning on can signify numerous system malfunctions, and difficulty starting the vehicle could be tied to issues with the battery or ignition system, rather than a clogged air filter. Thus, the drop in fuel efficiency directly correlates with the condition of the air filter, making it the correct answer.