Which of the following symptoms may indicate a problem with an HVAC system?

The presence of unusual sounds or odors during the operation of an HVAC system can be significant indicators of underlying issues. Unusual sounds, such as grinding, squealing, or rattling, may suggest mechanical problems with components such as the blower motor, compressor, or ductwork. These irregular noises can signify wear and tear, loose parts, or even blockages that could hinder system performance.

Similarly, strange odors can signal various mechanical issues or system malfunctions. For example, a musty smell might indicate mold growth within the ductwork, while the smell of burnt wires could point to electrical problems that pose a fire hazard. These symptoms require immediate attention to prevent further damage to the system or health hazards for occupants.

In contrast, the other options do not necessarily indicate HVAC system problems, as constant high outside temperatures, visibility of outdoor surroundings, and normal air pressure differences are typical environmental conditions or system characteristics that don’t inherently suggest malfunction. Recognizing the significance of sounds and odors in HVAC operation is essential for maintaining the system's efficiency and ensuring safety in the environment.