Diagnosis
That's the sound of the drive socket in the power base slipping against the blade assembly's drive gear. It happens when the load on the blades is too great for the motor's torque.
How to Fix It
1) Stop using it immediately when you hear that noise. 2) Unplug the base and inspect the drive socket (the raised, toothy plastic piece in the center of the base). Look for worn-down, chipped, or melted plastic teeth. 3) Inspect the corresponding socket on the underside of your blade assembly for similar damage. 4) The fix is to replace whichever part is damaged. Usually, it's the cheaper blade assembly that goes first. Going forward, don't try to blend super-hard items like whole coffee beans, ice cubes made in a thick tray, or packed frozen fruit without ample liquid.
If That Didn't Work
If you've replaced the blade assembly and the grinding persists, the internal gears in the power base are likely damaged. At that point, the whole base unit is a loss. It's not user-serviceable.
Heads Up
That grinding noise is doing real mechanical damage every time it happens. It's a clear sign you're exceeding the blender's design limits. Use the pulsing method and cut ingredients into smaller pieces.