What is wrong with my Car?

2017 Hyundai accent with only 37 thousand miles all of a make a funny
rumbling rough idol when going from park to reverse. Did this for a couple of
ays. Then all of a sudden idoling perfect & no problem backing up. Then
when I drive forward after first stop the car drags and then kicks into gear.
Drives perfectly fine until I stop at the next stop sign or light and again
acceleration is not performing correctly. It did this for 5 days until finally
engine light came on. Firestone didn't want to touch the car thinking
Transmission issue. My car in the hyundau dealership a week now.
Transmission being flush as no codes gave them no answer as to what is
wrong???