1.Grinding noise was Starter and / or Flywheel Gears "Having their way with each other" to the point that the Flywheel/Starter Gear interface is now "Locked Up".
[Which is why you can't turn the motor by hand]

2.You may have had a bad Flywheel Tooth(s) and the Starter Gear tore it up.
(Or Vice Versa)

3.OR (Shudder) Engine was already locked up, and the Starter self destructed
trying to spin the motor.

Never ANY good to be had,repeatedly attempting to use/activate a Starter
system making ANYTHING other than normal Happy engagement noises.
(The Amp Load created by "Grinding" noises from a Starter should activate
an excruciatingly loud 24" Bell under the dash.)

'Set emergency brakes and chock both rear wheels,fore and Aft.
'Jack up each side enough to place ramps,securely, under the front wheels.
Investigate.

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'84 300SD sold
124.128