Diagnosis
That's electronic noise, likely from the camera's power supply (the wall plug transformer) being picked up by the sensitive microphone. It's a known issue with some units.
How to Fix It
The first and easiest fix is to swap the power adapter. Unplug the current one and try a different 5V DC power adapter with the same polarity (center-positive) and a plug that fits. The camera uses a standard barrel plug. Often, a higher-quality or just different brand adapter eliminates the whine. Also, try plugging the camera into a different outlet, preferably one on a different circuit from fridges or dimmer switches.
If That Didn't Work
The noise could be internal. Try turning down the microphone sensitivity on the parent unit. Go to Menu > Settings > Audio and reduce the 'Sensitivity' or 'Volume' level for that camera. Sometimes the gain is just set too high, amplifying circuit noise.
Heads Up
If the noise changes pitch or starts clicking, that could indicate a failing component in the camera itself. If a new power adapter doesn't fix it, the camera likely needs service.