On my older car I would hear a winding noise. What would happen on a modern car?
My car doesn't do anything if I turn the key again after the engine starts.
On a modern car nothing will happen. The starter circuit is bypassed when the computer senses that the engine is running.
Horrible grinding noise - do not do it.
It will BREAK the starter and flywheel if you keep doing it. Do NOT do it.
The "winding noise" you hear is the gear from the starter motor grinding against the flywheel. That is a BAD THING. I would expect that a "modern car" would have the same reaction.
You would grind the gears between the starter and flywheel.
Many new vehicles will lock out the starter while the engine is running to protect the starter and flywheel ( or ring gear ).
You grind the starter gear and flywheel teeth down until they won't engage anymore. Then the motor and/or transmission has to come out to replace the flywheel.
On my smart Fortwo, if I try to start with the engine running it just ignores me.
You grind the teeth off your starter and fly wheel...
Just an unpleasant noise.