Hi, I've used this forum before with great results! I have a '91 s-10 4.3 L and my alternator is making noise. I think it's the bearings. It still reads an output of 14.67 volts. How do I tell when I really need to replace it. Thanks! Kevin

When you can hear it, it's time. I'd pop the belt off and turn the pulley to be sure that's what is making the noise though. Bearings can be replaced, but if high mileage, might be best to replace the unit.