I just had simular problem. Was my "brush pack" as earlier poster stated, philips screw driver is all that's needed, pack is on back of alternator. Tight fit under car though and not sure if you can reach from top. Might need a floor jack.
Brush pack for mycar was about $60, some say should replace whole alternator because brushes probably went bad due to bearing or other wear related issue, I had 120,000 miles and have had no problem since replacing brush pack.