My starter grinds when I try to start

Depends on the rest of the symptoms. Does this happen every time? Does it occur while the engine is cranking, or does the engine not spin?

If you can get it to occur continuously or repeatedly, rotate the engine by hand a little in the normal direction. If it still occurs, the best suspect is the shaft the starter drive rides on is rusty and needs to be cleaned up and lubricated. If the grinding never occurs right after you rotate the engine, a better suspect is worn teeth on the ring gear. That is often accompanied by slightly worn teeth on the starter drive gear too. This gets confusing because replacing the starter with the new drive gear it comes with can overcome the grinding for a little while, but it is never a permanent fix. Think of wearing a worn shoe and a worn sock. Replacing either one can keep you from feeling the rocks you are walking on, but the proper repair is to replace both. The good teeth on the new starter's drive gear can overcome the affects of the worn teeth on the ring gear, for a little while, but in the end you will have repeated failures. This is the confusing part. People mistakenly assume it is the starters that keep failing when they have never addressed the bigger problem; that ring gear.

Rebuilding the starter is not the solution because obviously it is working. New brushes and bushings will not make it work any better. The drive assembly can be defective too. That can be replaced, which is different than a rebuild, but considering the cost of the parts and the time involved, the best approach is to just buy a professionally-rebuilt starter with a warranty.