Which of the following is an indication that a steam boiler is experiencing a malfunction?

Abnormal noises coming from the boiler are indeed a significant indication that a steam boiler may be experiencing a malfunction. These noises can manifest in various forms, such as banging, popping, or hissing sounds, which often point to underlying issues within the boiler system. Such sounds can result from factors like the presence of steam bubbles or water hammer effects, both of which can compromise the efficiency and safety of the boiler. Identifying and addressing these noises promptly can prevent further damage and ensure the system operates safely.

In contrast, while consistent high pressure readings might also be concerning, they can sometimes occur within regulated operating parameters and do not necessarily indicate a malfunction without accompanying symptoms or context. Regular water level fluctuations can occur in normal operating conditions due to changes in steam demand, and efficient steam generation is a sign that the boiler is functioning correctly. Therefore, abnormal noises are more directly tied to potential malfunctions that need immediate attention.