Most likely the rear mode control motor. Common issue and probably even a trouble code for it in the HVAC system. Check to see if you can control the air in the back. Change the settings to get air from the ceiling and then to air from the floor. I'd be willing to bet it's stuck on one or the other.
I had that problem and after a bit I started hearing a squeak too. Had my husband take the panel off &
Turned out, there's a wheel with a hampster running on it in there!! His wheel needed an adjustment, and he needed a little food. 😉😉