Diagnosis
The drive screw inside the rail is dry or the trolley's drive nut is worn. The RJO101 uses a steel screw, not a belt. Lack of lubrication causes metal-on-metal grinding.
How to Fix It
Get a tube of Chamberlain garage door opener lubricant. Apply a bead along the entire length of the exposed drive screw. Run the opener up and down twice to spread it. Do not use WD-40 or grease.
If That Didn't Work
The drive nut on the trolley is likely stripped. You'll see plastic debris in the rail. You must replace the trolley assembly. Part # is 41A4277 for the RJO101.
Heads Up
Grinding on the down cycle only often means the door is too heavy for the force setting. After lubricating, check and increase the down force slightly via the adjustment dials if needed.