Starter and flywheel are grinding

I'd have to visually check your starter and flywheel, see if the flywheel has any missing or badly worn teeth. If it is problem with bad flywheel teeth, sometimes you can rotate the flywheel to get the bad spot away from starter, then it will crank normally. Of course that doesn't fix the problem. Check the starter drive gear, is it badly worn? Some makes and models, you can use shims at the starter to get starter and flywheel teeth, to mesh properly. My biggest problem is I can't see anything from here.

The old starter may have worn the teeth down on the ring gear and now the new starter is starting to slip, remove the starter and inspect the teeth on the ring gear first.

Thanks a lot for the advice. I'll pull the starter out and check it out.