Try checking the mud guard on the rear wheel, not the one on the back of the wheel, but the small one in front of the wheel. It could be one of the inner yellow clips is not snapped in all the way and causes to rattle when driving.
I have a 2013 Santa Fe Sport. I've had it about a year and about 6 months ago I began hearing a 'clicking' sound. It sounds like it's coming from the back driver's side door area. I took it to the dealer and they pulled the panel off the back door and found nothing loose but they also couldn't duplicate the noise. Of course when I picked it back up from them the noise was still there so I made one of the maintenance guys ride with me until he heard it too. He thought it was probably from the bottom panel on the exterior door being loose. They have replaced both the front and back door on the driver's side plastic panel at the bottom of the doors but I can STILL hear this 'clicking/rattling' noise. It's mostly at high speeds on the highway, but can be heard when I drive around town too. Any ideas? The guys at the dealer don't seem to have a clue what could be causing it.