Which of the following is a common symptom of a malfunctioning pressure regulator?

A common symptom of a malfunctioning pressure regulator is fluctuating downstream pressure. This occurs because the regulator is designed to maintain a steady output pressure despite variations in upstream pressure or changes in flow demand. When the regulator fails, it may not be able to respond properly to these changes, leading to unstable pressure levels downstream. Such fluctuations can impact the performance of the entire hydraulic or pneumatic system, leading to inefficiencies, rough operation, or even damage to downstream components.

Constant pressure regardless of flow, excessive noise, and temperature fluctuations, while they can be indicative of other issues within a system, do not directly correlate with the specific failure of a pressure regulator in the same way that fluctuating downstream pressure does. Thus, the primary and most relevant symptom indicating a problem with the pressure regulator is indeed the fluctuation of pressure downstream.