So my 2011+ developed a pretty frequent rattle that consistently occured at highway speeds. It sounds like it was coming from somewhere around the passenger side dash area. There is another thread where several people talked about a similar issue, but they all located the culprit as the map lights. After a lot of experimenting pushing around the light, that didn't seem to be the issue for me.
I took the car to the dealership this morning and they are saying it is the glove box assembly and are ordering me a new one under warranty. It seems very likely that they are correct in identifying the problem as it sounds like the rattle is coming from the dash/glovebox area and it does sound like a plastic rattle...not like something mechanical the under the hood. However I have tried pressing/bracing various areas of the glovebox and could not conclusively determine that the glovebox is creating the rattle, so I'm still open to the possibility that the Kia tech was not correct.
I will come back and update this thread once the new glove box assembly is installed (dealership had to order the part). But just wanted to open this thread in case anyone else is having the same issue.