Four years after Hyundai replaced the engine in my 2014 Sonata under a recall program, an intermittent rumbling sound is coming from the GDI replacement engine. This only happens when I am driving the car, or when I'm backing it out. There is NO RUMBLE when the car is (i) idling, (ii) in Park and I rev the engine or (iii) in Drive with my foot on the brake. The rumble can only be heard from inside the car, not by others standing outside the car. There are no dashboard warning lights on, and the diagnostic scanners have shown no diagnostic trouble codes. Other than the rumble, the engine seems to be running great with only 30,000 + or - miles on it (72,000 miles total, between the old engine and the replacement engine). I've been to two different Hyundai dealers and a top notch local mechanic's shop attempting to get this problem resolved. I was advised to have the motor mount, transmission mount and tensioner replaced. I dd so (to the tune of $2,000) yet the problem persists. I've been on YouTube trying to diagnose the source of this rumble sound, but none of the engine noises addressed in these videos applied to my situation. I would appreciate any and all ideas you could send my way to help diagnose the source of this worsening rumble. TIA!