I'd bet money that's the door actuator for the HVAC modes. Mine was easier to identify because if you switch to floor vent it would go away, but every other mode it was there.
Actually had the dealership flush my radiator (paid for it was due anyway, 50,000 miles) then went back and asked them to flush it again and get the air burped out of it. I thought the sound was gurgle from air in the heater core. They kind of heard it, but didn't ever identify it. That day I had the second flush I was about to drive off, and the noise was present (drove it it was dormant, it came and disappeared often) Pulled the service adviser he pulled the tech, and we all sat in the car fukin' with it until we could narrow it down, he had his head under the driver side dash at one point, and we were drilling through all the settings, and he could either see or feel it, not just hear it.
They had the part on hand, took about an hour to swap it out, lot of contorting. Irony was i had to go back because he didn't connect it right, and all i had was air out of the bottom vent no matter the mode. Still not sure about the level of detail at the kia dealerships, wasn't the first time i had to go back for something.
The sound was gone for about a year, it's unfortunate that the sound has come back, just a bad part, or it's in a bad spot that it gets damaged.