DIAGNOSIS The drive chain is likely dry or has come out of alignment with the rail sprocket, causing metal-on-metal contact.
FIX Inspect the chain where it engages with the drive sprocket on the motor unit and the idler sprocket at the far end of the rail. Look for shiny, worn links. Apply a dry lubricant specifically designed for garage door opener chains to the entire length of the chain. Do not use wet grease or oil. Check that the chain is centered on both sprockets and adjust the rail alignment if it is riding off to one side.
IF THAT DIDN'T WORK The issue may be a failing gear inside the motor unit's gear assembly. Listen closely to see if the grinding is loudest at the motor unit itself. Gear replacement requires disassembly of the motor unit.
HEADS UP Before lubricating, disconnect power to the opener. Manually move the door trolley along the rail to expose the entire chain. Wipe off old lubricant and debris first.