DIAGNOSIS A low hum during heating is normal operation of the boil-dry protected heating element. A loud, sharp buzzing or rattling is not.
FIX First, ensure the unit is on a stable, level, and hard surface. Check that the thermal carafe is seated correctly on the warming plate. A loose carafe can resonate. If the noise persists, unplug the unit and check for any foreign objects underneath or inside the base.
IF THAT DIDN'T WORK The noise may be a vibrating internal component, such as a wire touching the housing or a loose screw on the base plate. This requires inspection by a technician.
HEADS UP If the buzzing is accompanied by a failure to heat properly, or a burning smell, unplug the unit immediately and discontinue use. This indicates a serious electrical fault.