bad alt,you can have it rebuilt only at napa or a starter alt rebuild shop.or get a new or rebuilt at auto store.to remove 2 bolts top and bottom.first unhook battery 2nd remove serpentine belt 3rd unplug,plug on top of alt look on back unhook wire.do reverce and reinstall rehook battery.

It's easier to replace the whole unit, and you should get a lifetime warranty at most auto parts stores on a rebuilt alternator.  I've done bearings, etc., in alternators and it's hardly worth the time, plus if the bearing is starting to go, you might as well assume the other parts are starting to go bad as well.  This could be your voltage drop.