DIAGNOSIS The rail trolley's plastic drive gear is likely worn out or stripped. This is the most common cause of grinding on these chain-drive models.
FIX Replace the plastic drive gear assembly (often called the gear and sprocket kit).
1. Unplug the opener and disconnect the emergency release rope.
2. Remove the light cover to access the motor head.
3. You'll see a large plastic gear on the motor shaft. If it has missing teeth or looks chewed up, that's the culprit.
4. Order the official LiftMaster replacement gear kit (part #41A2037 is common for this series). Follow the kit's instructions for the swap.
IF THAT DIDN'T WORK The noise could be from a dry chain or worn trolley wheels. Lubricate the chain with a dedicated garage door lubricant (not WD-40) and check the trolley wheels on the rail for cracks.
HEADS UP Continuing to run it with a stripped gear will burn out the motor. Replace the gear as soon as you hear grinding—it's a wear item designed to fail before the more expensive motor does.