What physical sign may indicate that a furnace is experiencing a fault?

Unusual sounds when operating can indicate that a furnace is experiencing a fault because these sounds often signify mechanical issues or irregularities in the operation of the heating system. For example, it could be indicative of a problem with the blower motor, ductwork issues, or even combustion problems within the burner assembly. If a furnace is functioning properly, it should operate quietly and efficiently.

The other choices represent signs of normal functioning or lack of issues. Consistent airflow from the vents typically suggests that the furnace is operating effectively and circulating heated air as intended. Similarly, warm air output is a normal sign that the furnace is heating the space adequately, and no changes in heat production imply that the furnace is maintaining its heating capability without any fluctuations, which means it is likely functioning correctly. Thus, the presence of unusual sounds is a reliable indicator of potential faults needing further investigation.