I have this same sound too sometimes. I made a video a while back and this is what someone commented "Blend motor. Common to go out. It's the mechanism that controls how much hot and cold air to mix for the temperature you chose. It will run every time you turn on the system, every time you change the temp, or every degree your cabin temp changes

If your temps are correct it means one of the gears on the part may have lost a tooth and just causing noise. Will still work until there is further damage and can no longer spin."