If the alternator on your 1999 Chevy Lumina is not keeping your battery charged or has started to make noise when the car is running, it is time to test and replace it. Metallic scraping or grinding noises coming from the alternator indicate a bad bearing assembly inside the case.
A consistently low battery may indicate the alternator or voltage regulator is failing. In either case, replacement of the alternator is the best option. Replacement alternators are available at most auto parts store and if you bring in your old one, they will often bench test it free.
Remove the negative battery cable from the battery. Loosen the retaining bolt and lift the cable off the battery terminal. Isolate the cable from the battery while you are working.