Which of the following is NOT a symptom of a malfunctioning boiler?

A regular combustion cycle is indicative of proper boiler operation, as it reflects that the boiler is functioning efficiently and maintaining a stable burn of fuel. In a well-functioning boiler, the combustion cycle occurs at a consistent rate, ensuring steady heat production and preventing potential issues.

In contrast, the other options represent clear symptoms of a malfunctioning boiler. Pressure fluctuations can indicate a problem with pressure control or safety devices. Unusual noises often suggest mechanical issues, such as cavitation or component wear. Leaks are a direct sign of potentially severe faults in the system that could lead to operational hazards. Recognizing the typical signs of a malfunctioning boiler helps in timely maintenance and prevents hazardous situations.