If the pump in a hydraulic system produces a low rumbling noise while in operation, this is a probable indication of __________.

Correct Answer: Option D — air passing through the pump

Air ingestion, or aeration, causes cavitation-like symptoms in hydraulic pumps. As air bubbles enter the suction side and collapse under pressure, they produce a characteristic rumbling or grinding noise. This indicates a breach in the suction line or low reservoir levels, violating standard hydraulic maintenance practices for system integrity.