Probably worn brushes in the fan assembly. You can try taking it apart and cleaning it, but I'm guessing it's replacement time. If you can find an "old school" automotive electrical rebuilder in your town... typically they specialize in alternators and starters... they might be able to fix it. Otherwise, just toss a new one on and you should be good to go for a while.
www.infinitipartsusa.com is a good source for new OEM parts. Or you roll the dice with used or aftermarket parts.
www.car-part.com
Heath