Any kind of insight is good

why replace the battery and alternator,they won't grind? pull the starter out and look at it,then take it to have it tested,and/or check where the starter mounts,look at flex plate,check it over..unless you want to give the DEALER the kind of money they charge,take it to a good,trusted,independent repair shop..

By what you have described it sounds as if you have a starter and/or flex plate issue. There is a gear on the end of the started that connects to the flex plate to start your engine. If the teeth are worn on the starter or the flex plate gears are worn or missing you could have a grinding noise when attempting to start your engine. I would have it checked out by a trusted shop!

I agree with previous answers..possibly the flex plate or even the solenoid connected to the starter.