Starter or flywheel
 

I just put a new used motor in my 94 k1500 and I started i for the first time today everything was great except when it was starting it made a loud sound from underneath kinda like it was grinding or something was hitting something else. The old motor did the same thing and the new one came with a flywheel on it so its not the same. So what could it be?

grinding
 

sound's like the starter is either wrong or need's to be shimmed ,was there any ground off teeth on the flywheel ?you better check the clearence on the starter to starter drive teeth.it might have been changed and they left the shim out .

Can I just leave it like it is or will it eventually cause something bigger to tear up?

you can let it go but eventually it will tear up the starter drive or knock off some teeth from the flex plate.

I'm having the same problem with my 98 K1500. I rebuilt the engine and put a new flywheel and starter. I have tried 2 different starters and tried it with a shim and without a shim. I get the same results.

Mine stopped doing it:D:D. I havent done anything to it i guess something broke or something. I love it when problems fix them selves and as long as it keeps working im not going to touch either one.