Which observation might indicate a faulty gearbox?

The presence of abnormal noise coming from the gearbox is a significant indicator of potential faults within the gearbox system. These noises can include grinding, whining, or clunking sounds, which suggest there may be issues such as insufficient lubrication, damaged gears, or bearing failures. Such sounds often arise when internal components are not functioning smoothly or are misaligned, signaling that the gearbox requires immediate attention to avoid further damage.

In contrast, while unusually high fuel consumption, inability to start the car, and excessive exhaust smoke can indicate various mechanical problems, they are not directly tied to the gearbox's performance. High fuel consumption may relate to numerous factors such as engine tuning or fuel injection issues. Inability to start the car is typically related to the starting system or battery. Excessive exhaust smoke could indicate engine-related problems such as oil burning or fuel mixture issues. These situations do not specifically point to the functionality or condition of the gearbox. Hence, identifying abnormal noises is the most direct and telltale sign of a gearbox malfunction.