Diagnosis
That's AC line noise from the triac-based temperature controller on the main board. It's a common electrical hum.
How to Fix It
Plug the iron directly into a wall outlet. Do not use a power strip or extension cord. The reduced current draw often quiets it.
If That Didn't Work
The sound is normal operation for some units. If it's accompanied by a burning smell or flickering lights, unplug immediately. The main board is failing.
Heads Up
This buzzing is not dangerous by itself. But if the sound changes to a loud pop or crackle, stop use. That indicates arcing.