The ring gear around the flex plate (torque converter) could be damaged. Pull the dust cover off the bell housing and examine the gear as you turn the engine by hand. You may want to use a prybar or other device to assist you. If you see an area of the gear that is worn through or nearly worn through, you'll need to replace the gear. The flywheel is probably damaged. The flywheel has a gear rung around the outside which is where the gears on the starter engage to start the engine. It is possible that you did not put the proper shims in when you installed the starter as sometimes the alignment is slightly off which could also cause the grinding noise. Make a video of the noise and put it on You Tube and ask people what it sounds like. VBdenny
Mounted to the top of the starter
How do you install a starter in a 1977 Chevrolet Caprice Classic with a 5.7 liter motor?
Look at the bottom rear of the engine
Isn't it attached to the solinoid mounted to the starter? vbd
Passenger side, mounted on the bell housing, connected to the flywheel.
Loose, defective, or corroded cables, loose ground wire, starter, or starter solenoid.
replace the starter
Disconnect the battery. Remove the wires from the starter then unbolt it. Lower the starter, then fit the new one in. Bolt up the starter and connect the wiring. Reconnect the battery.
The only 100% sure way to know is to remove the starter and have it bench tested at an auto parts store.
Under the passenger side rear of the engine, just follow the positive battery cable from the battery, it will lead you right to the starter.
Disconnect the battery, locate the starter.. remove the mounting bolts, bring the starter down to where you can reach all of the wires going to the solenoid, and disconnect those.
You need to go to a parts house and buy the bolts. They are special in the fact that the shoulders are made to keep the starter straight.