I have a 12 veloster NA AT a few months ago I took it to the dealer to be looked at it had a jackhammerish noise going up hills and when I would turn the car off. They said I need a new transmission and the warranty wouldn't cover it because I skipped getting the air cabin filter changed. Yesterday it started making that sound louder and happened between 600 rpm and 2000 rpm while accelerating. I'll try getting an audio clip soon it's probably too windy today but imagine a the sound of sitting in your car while someone is using an impact wrench to remove your tires. Also you can feel it in the petal if that helps.