If you have a late model GM car, you're going to have a starter with a reduction gear AND a permanent magnet motor. In the 1960's, Chrysler introduced this concept of gear reduction in a starter on a high volume basis (Rolls Royce was the first). In the mid 1990's to the best of my knowledge, car makers began adopting this technology and using permanent magnets. The result is a much more efficient and much lighter starter that uses less materials, particularly copper. On the downside, these are much more fragile than the old school starters (mainly the ones I grew up with) and they're a helluva lot more expensive. You can still get one for a 1977 Olds Delta 88 (my first car) rebuilt for about $50 and some change. For my current car, which hits 10 years old in September, this part can exceed $200! Unlike the former, this one is not rebuildable to a reliable standard using common tools. Unless you have access to specialized equipment and can get the parts, you won't be able to do this. Even if you get it apart, you will utter obscenities getting this back together. I know because I tried it. Yes, I got it back together and it seems to work fine, but had to use alligator clips to hold the brushes while getting the armature back in.
Truth is, you can take this apart and get it back together successfully to at least get an inkling of what's wrong with it. Mine started acting funny between a failing battery and a dunk in the mud (I still blame you Walmart!) After replacing the battery, it started fine for two days and the weather got colder. The result was a starter that clicked smartly, but would not turn. I retested the battery and it was fine, but alas no crank. When the mercury drops below a certain point, car repairs are deferred. I used to fix my jalopy in all kinds of weather short of an electrical storm when I was a teenage kid. In my early 40's, this isn't happening and besides, working 12 to 17 hour days when furnaces break down leaves little time for repairs, much less sleep.
If for some reason, your engine gets wet or muddy expect your starter to act up. I washed the underside pretty aggressively after slipping off an exit ramp and getting mud all over the underside. It didn't help that the gasket was twisted up between the field frame and the aluminum nose cone either, possibly letting water in. Once it was able to stay in the warm house and freeing up the drive with a pair of pliers, it bench tested fine.
When you take it apart, you're looking for a burned smell, something physically damaged, or something that doesn't belong inside like mud or water. Don't even bother taking the armature (that round part that spins) out of the field frame (the steel tube it lives in) due to the issue with the brushes. You can remove the bolts, separate the field frame from the nose cone and inspect the gears inside making sure they have enough grease. You can try grabbing the drive with pliers (don't chip those teeth) and spinning it up. That's all you can do to service this thing. If it doesn't work after trying these steps, you need to replace it. Whatever you do, even if you think it is toast NEVER and I mean NEVER try to hit this or any late model starter with anything to unstick it, period. The permanent magnets inside are expensive to replace for a re-builder and may result in a loss of a core refund. In short, it will turn this starter into a groovy and expensive paperweight. They're made of ceramic and WILL and I mean WILL BREAK. I suppose you can take the solenoid off this one too, but they aren't available anyway and you'll need a 4mm deep well socket to get them off. My advice is to leave it on, because if this is failed you'll have to replace the starter anyway. Maranatha!
UPDATE! The starter is junk. After no less than ten bench tests, it's doing the same old same old and not rotating. Gotta bite the bullet and replace it. As I've found, this is un-rebuildable with common tools and I've been unable to find parts for these. Even with the old school direct drive units, success was pretty low for this handyman. Unlike the old 10SI alternators that are pretty easy to rebuild (these are on 1970's to mid 1980's GM vehicles) successfully, starters are not my forte'. Oh well!