Diagnosis
That's the internal cooling fan running continuously, which is not normal for this model. It indicates the unit is overheating.
How to Fix It
This is a hardware flaw requiring a replacement. The fix is to get a new unit from Eufy. Steps: 1) Note the serial number in the app. 2) Record an audio clip of the buzz. 3) Contact Eufy support via the app with the evidence; they’ll process a warranty swap.
If That Didn't Work
In rare cases, a firmware bug can cause the fan to stay on. Try a hard reset: hold the sync button on the unit for 10 seconds until you hear a prompt, then reconfigure in the app. If the buzz persists, it’s definitely hardware.
Heads Up
Don’t ignore this. A constantly running fan points to a failing thermal management system, which can shorten the lifespan of the internal components dramatically.