DIAGNOSIS: That's usually something interfering with the cooling fan, which is located at the back of the unit. A loose piece of food packaging, a stray grain of rice, or a buildup of dust and grease can cause this.
FIX: Unplug the cooker and let it cool. Tilt it forward and look at the vent slots on the back. Use a can of compressed air (like for cleaning keyboards) to blow gently into the vents to dislodge any debris. You can also use a vacuum cleaner hose with a brush attachment on a low setting to suck dust out from the vents.
IF THAT DIDN'T WORK: If the noise is a consistent grinding, the fan motor bearing might be failing. This isn't a user-serviceable part. You'll need to contact Ninja for service or a replacement.
HEADS UP: Keep the area around the back vents clear when using the cooker, especially during air frying or pressure cooking when the fan runs a lot. Good airflow prevents overheating.