Diagnosis
The drive sleeve is failing. The screw drive passes through a nylon drive sleeve inside the motor unit. When it wears out, the motor gear grinds against the sleeve instead of turning the screw smoothly.
How to Fix It
You must replace the drive sleeve, part #GTR-DSM. Unplug the opener. Remove the light cover and the two screws securing the motor cover. Slide the cover off. You'll see the white nylon sleeve on the end of the screw drive. Pull it straight off. Lubricate the new sleeve lightly with Genie Silicone Lubricant (not grease). Press it onto the shaft until it seats fully. Reassemble.
If That Didn't Work
If the noise persists, the motor pinion gear itself is likely worn from running on the bad sleeve. That requires replacing the entire motor assembly.
Heads Up
Do not run the opener with this noise. It will destroy the motor pinion gear. The repair is cheaper than a whole new motor.