Yeah, this one's common. It's usually the heating elements expanding and contracting as they heat and cool. A few faint clicks at the start or end of a cycle is normal operation.
If the clicking is loud, persistent, or sounds like something is loose, stop the cook cycle. Unplug the unit and let it cool completely. Check that the nonstick baskets and crisper plates are correctly seated and not warped. Make sure nothing has fallen into the air intake vent on the back or the air outlet.
If that didn't work, and you hear the noise every time or it's accompanied by smoke, that's not normal. Unplug immediately and do not use it. Contact Customer Service.
Heads up: Never cover the air intake or outlet vents while the unit is operating. It needs that airflow to function safely and will overheat if blocked.