DIAGNOSIS Yeah, this one's common. That grinding is almost certainly something—usually a small piece of food or a bone fragment—that got past the basket and is now hitting the fan blades inside the heating chamber.
FIX Unplug the unit and let it cool completely. Remove the drawers and baskets. Shine a flashlight into the heating chamber at the top. You’ll see the fan. Use a pair of long tweezers or needle-nose pliers to very carefully remove any visible debris. Don’t touch the heating element. Rotate the fan by hand (gently) to check for anything stuck on the backside of the blades.
IF THAT DIDN'T WORK If the noise persists with no visible debris, the fan motor bearing might be failing. Listen closely—a consistent bearing whine or grind points to a hardware fault. You’ll need to contact Ninja support at that point.
HEADS UP Always double-check that smaller foods like french fries or chicken wings are fully inside the basket and not dangling through the holes. A single stray piece can cause this racket.