DIAGNOSIS That's likely a worn drive gear, specifically the plastic gear inside the motor unit's gear train. Under load (starting to move the heavy door), the worn teeth slip and grind for a second before catching.
FIX This is a repair you can do. You'll need a replacement gear kit (often called a "rail saver" kit) for Chamberlain/LiftMaster belt-drive models. 1) Unplug the opener and disconnect the emergency release rope. 2) Remove the light cover and the main cover. 3) You'll see the motor and gearbox. The kit has instructions, but essentially you're removing the old plastic gear and replacing it with a new one, often including a updated steel worm gear. It's about a 30-minute job.
IF THAT DIDN'T WORK If the noise persists after the gear replacement, the issue might be in the rail itself. Check where the trolley attaches to the belt. Make sure the bolts are tight and the trolley isn't cracked, causing a slight bind at the start of travel.
HEADS UP Don't ignore this. That plastic gear is designed to fail to protect the motor. If you keep running it, the grinding will get worse and you'll strip the gear completely, leaving the door stuck shut.