Diagnosis
That's the drive gear stripping. The plastic gear inside the motor unit is worn down and slipping against the metal worm gear.
How to Fix It
The gear assembly needs replacement. Order the Chamberlain part #41A2033, which is the gear and sprocket kit for the B970T. Disconnect power. Remove the light cover and light bulb. Unbolt the cover. You'll see the gear train. Remove the old gear and sprocket, clean any old grease, and install the new kit. Apply the included grease to the worm gear.
If That Didn't Work
If the new gear grinds immediately, the motor itself is seizing. The motor bearings are shot. That requires a full motor unit replacement, which often means replacing the entire opener.
Heads Up
Do not run the opener with a stripped gear. It can overheat the motor and cause permanent damage. Replace the gear as soon as you hear the grind.