Thanks for the response. I'm a dinosaur and my default setting goes back to the '50s, '60s and '70s when I understood what went on under the hood. Enjoy your Mach1.
If the current owner is an old burn-out like me (79 yrs. old). He might remember how back in the old days if you didn't change the oil every three months you would end up with sludge in the crankcase and eventually develop the wonderful sound of lifter rattle. I have an '18 and '24 GT and...
Try revving the engine just a bit before you put it in gear and see if this eliminates it. I have an '18 GT that does this, and tapping the accelerator eliminates the hard shift.
Have a 2018 GT with 10 speed auto (think this is the first year they switched from 6 speed auto) and when you go directly from Park or Neutral to Drive there is a significant jerk. I don't know if these modern transmissions have a flywheel, but that's what it feels like when back in the good old...