There's a bearing in our transmissions that blows up. Stop driving the car, you have metal shrapnel floating around inside it.
details and repair fix:
I finally got that P0734 4th gear incorrect error fixed. Here's what I did - Hyundai Forums : Hyundai Forum
Drop the trans pan and look for metal suprises before you go any farther. It takes about 2 quarts to refill after a pan drop.