Noises that come from the AC generator can be from three sources. The causes of the noises are identifiable by the type of noise they make. A loose belt will make a squealing noise. Check the belt condition and tension. Replace the belt if necessary.
A squealing noise can also be caused by faulty bearings. The bearings are used to support the rotor in the housing halves. To test for bearing noises, use a length of hose, a long screwdriver, or a technician's stethoscope. By placing the end of the probe tool close to the bearings and listening on the other end, any bearing noise will be transmitted so you will be able to hear it. Bearing replacement will require disassembly of the AC generator.
This test is performed with the engine running. Use caution around the drive belts, fan, and other moving components.
A whining noise can be caused by shorted diodes or stator, or by a dry rotor bearing. A quick way to test for the cause of a whining sound is to disconnect the wiring to the generator. Then start and run the engine. If the noise is not there, the cause of the noise is a magnetic whine due to shorted diodes or stator windings. Use a scope to verify the condition of the diodes and stator. If the noise remains, the cause is mechanical and probably due to worn bearings.