Diagnosis
The drive coupling on the bottom of the pitcher is damaged or has debris lodged in it. This transfers abnormal force to the motor shaft.
How to Fix It
Unplug the blender. Remove the pitcher and turn it upside down. Inspect the black plastic drive coupling (the star-shaped piece). Look for cracks, chips, or food particles jammed in the teeth. Clean it thoroughly with a dry brush. Check the metal drive socket on the base for debris. Reassemble and test.
If That Didn't Work
The drive coupling may be stripped. You need a replacement pitcher base. Order part #TB301P from Ninja. It's the entire plastic assembly that holds the blade. It twists off counterclockwise.
Heads Up
A grinding noise under load often means the coupling is already failing. Continuing to use it can damage the motor shaft, which is a base replacement.