Diagnosis
That's the internal fan running, and it's a normal sound for this model when it's working hard. The TrackMix has active cooling because the 4K dual lenses and PTZ mechanics generate heat.
How to Fix It
You can't turn the fan off, but you can check if it's running excessively. The fan spins up based on internal temperature. Ensure the camera isn't in direct, blazing sun all day, as that heats it up more. Also, make sure the vents on the camera body aren't covered by a spider web or debris. A quick blast of compressed air (with the camera off) around the seams can clear dust from the intake.
If That Didn't Work
If the noise is more of a grinding or clicking sound, not a steady whir, that's different. That could be a gear in the PTZ mechanism. Try manually panning and tilting the camera through its full range via the app to see if the noise changes or if there's a spot where it grinds.
Heads Up
The fan is noticeably louder when the infrared (IR) night vision is on, because the IR LEDs produce significant heat. If the noise bothers you at night, you could try using a nearby porch light instead of the IR, or adjust the IR intensity settings (Settings > Display > IR Light) to a lower level, though that will reduce night vision range.