DIAGNOSIS That's usually the fan housing or the fan itself vibrating against something. The most common cause is the crisper plate not being seated flat in the bottom of the cooking pot.
FIX 1) Turn off and unplug the unit. Let it cool. 2) Remove the cooking pot and take out the crisper plate. 3) Look at the four little feet on the bottom of the plate. Make sure none are bent or missing. 4) Place the plate back in, ensuring it sits completely flat and level on the supports. There should be no wobble. Run it empty for a minute to test.
IF THAT DIDN'T WORK The noise could be coming from the main fan behind the rear vent. Carefully use a vacuum hose attachment to suck out any crumbs or debris from the vents. Something small like a popcorn kernel caught in the fan can cause a racket.
HEADS UP A low hum from the fan is normal. A loud mechanical rattle or buzz is not. Don't ignore it, as a severely off-balance fan can wear out the motor bearing.