I have had the exact same issue with my 09, starting about 1 week ago. I took it to the dealership since it still under warranty and had them check it out. The said there was a TSB to update the transmission, i guess through the ECU, they said they drove it and didn't notice it. Well I got in it and this morning and the same problem, only when the tranny shifts from 4 to 3. I only have 47k on mine, but called the dealership back up and they suggested getting the trans fluid drained and flushed and if it keeps having the issue then call them back up.
Let us know if the trans service helps your issue, I am going to be getting mine done in the next few weeks, I dont drive the Sonata everyday, i drive an older subaru to keep the miles low.