Cavitation/ Air leak - An air leak in the suction line can generate air in the pump which leads to load cavitation (air in the upper part of the strainer basket). Cavitaion can also be generated when the discharge line has less resistance (e.g. larger pipe diameter) than the suction line. You may have to throttle the discharge line.
Bad bearings - One of the most common sources of motor noise. When the bearings get wet or worn, they will generate a loud squeal until they fail. A common cause of bearing failure is a worn shaft seal that sprays water onto the motor and its bearings. Replace the shaft seal when you replace a motor.