The oil level in the pump is low.

Add power steering fluid as required. Refer to Checking and Adding Power Steering Fluid .

The pump belt is slipping.

Refer to Drive Belt Replacement in Engine Mechanical 4.3L.

There is high internal leakage in the steering gear or the pump.

Refer to Power Steering System Test .

The drive belt tensioner is weak.

Refer to Drive Belt Tensioner Diagnosis in Engine Mechanical 4.3L.