DIAGNOSIS That's coil whine from the main heating element or its electrical components. It's a common, though annoying, trait in some units and isn't typically a sign of failure—just cheap component vibration.
FIX First, ensure the unit is on a perfectly level, solid surface. Vibration on a countertop can amplify the noise. Place a non-slip silicone mat or a folded towel underneath to dampen resonant vibration.
IF THAT DIDN'T WORK The noise often diminishes after the first few uses as components 'settle.' If it starts suddenly after months of quiet use, it could indicate a loose screw on an internal bracket or fan guard. Unplug, let cool, and check for any obvious loose parts you can access (do not open the sealed housing).
HEADS UP If the buzzing is accompanied by a burning smell, sparks, or the unit cutting out, stop use immediately and unplug it. That indicates a serious electrical fault.