Diagnosis
That's electrical coil whine from the camera's internal power components. It's a known issue with some units and gets more noticeable in a quiet room at night.
How to Fix It
Unplug the camera's power adapter from the wall for 30 seconds and plug it back in. Sometimes a power cycle clears it. If it persists, check if the noise changes when you slightly wiggle or adjust the power cable where it plugs into the back of the camera—a poor connection can sometimes cause it.
If That Didn't Work
The power adapter itself might be the source. Try using a different 5V power adapter with the same plug size (center-positive, barrel plug). A higher-quality adapter often eliminates the whine.
Heads Up
This noise is usually a nuisance, not a sign of impending failure. If it's new and unbearable, contact HelloBaby support, as some users have reported replacements for persistent, loud coil whine.