Causes of Abnormal Noise in Hydraulic Pumps and Control Methods

Pumps in hydraulic power systems—including gear, piston and axial piston series—naturally produce a certain sound level during circulation because of their operating principles. Abnormal noises that depart from the system’s usual rhythm or change in character, however, are among the clearest acoustic signals that a problem is beginning in the hydraulic circuit. For an experienced maintenance technician or field engineer, the sound emitted by the pump is a powerful diagnostic tool that can help isolate whether the source is cavitation, air ingress or a mechanical fault before the system is dismantled.

Three Main Categories of Abnormal Sound and Their Mechanical Meaning

Pump noises can be examined under three principal headings according to their acoustic characteristics:

1. Cavitation Symptom: “Gravel / Metallic Rattle” Sound

Cavitation occurs when the absolute pressure at the pump suction port falls below the vapour pressure of the hydraulic oil. Microscopic gas or vapour bubbles form in the fluid; as soon as they enter the high-pressure region of the pump, they collapse within milliseconds. These micro-implosions generate a strong sound wave in the system.

• Sound Character: A hard, continuous, high-frequency rattling that sounds as though gravel were being poured into the pump or dry sand were circulating through it.
• Primary Causes: A suction hose that is too narrow or too long, restrictions in suction-line fittings, a tank outlet valve (T valve) that is not fully open, oil that has become excessively thick in cold weather, or a clogged suction filter. Bubble collapse can remove microscopic particles from metal surfaces, creating pitting damage and gradually wearing the pump.

2. Air Ingress Symptom: “Hum / Irregular Grinding” Sound

Air ingress, or aeration, differs from cavitation in that atmospheric air is drawn directly into the system through leaks in the suction line or because the oil level in the tank is too low.

• Sound Character: A deeper, irregular and sometimes intermittent humming or grinding sound. When air is being drawn in, foaming is visible in the hydraulic oil tank, system pressure becomes unstable and the cylinder stages extend with jerky, interrupted movement.
• Primary Causes: Loose suction-hose clamps, worn pump-shaft seals, or an oil level below the minimum limit that creates a vortex in the tank and draws in air.

3. Structural Mechanical Failure Symptom: “Rhythmic Knocking / Metallic Rubbing” Sound

This is the most critical noise category and occurs when mechanical parts inside the pump have suffered structural damage.

• Sound Character: Rhythmic knocking, cracking or heavy metallic rubbing synchronised with engine speed and pump-shaft rotation. The noise becomes louder as the system is loaded.
• Primary Causes: Bearing damage, deformation of gear surfaces, a bent pump shaft or a large foreign particle entering the system and scoring the internal mechanism. The system must be stopped immediately in this condition; otherwise, internal damage may worsen and metal debris may spread through the hydraulic lines, damaging cylinders and valves as well.

Acoustic Diagnosis Summary and Response Matrix

Conclusion

The sound of a hydraulic pump is a live indicator of system health. Keeping the oil warning/weep holes specified in pump catalogues clean, observing suction-line tolerances and correctly analysing abnormal sounds at an early stage are among the most effective operational monitoring practices for preventing a condition that could be resolved by simply tightening a hose from developing into major system damage.