Try to turn the compressor by hand. It should be possible to turn it around. It should turn around smoothly, without any 'hickups' (be sure to completely shut down the air compressor!). Check the isolation of the motor windings.
[pdf] This Operation and Maintenance Manual describes in detail the safety precautions, structural functions of each system, and operation & maintenance method for Model 900XH, 980RH, 780RH, 780VH, 850RH portable screw air compressors driven by CUMMINS QSM11 diesel engine designed and produced by Sullair.
[pdf] Rotor clearance issues or debris can lead to metallic contact sounds Loose or worn belts squeal or howl, especially under load Airflow pulsations, valve leaks, or turbulence create hissing or whistling Bolts, nuts, and mounting pads loosen over time, amplifying noise
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