Diagnosis
That initial grind is usually a lubrication issue, not a motor problem. The opener's drive gear is engaging the trolley (the part that connects to the door), and if the trolley's rail or the gear teeth are dry, you'll hear metal-on-metal scraping for a second.
How to Fix It
Unplug the opener and disconnect the door from the opener by pulling the red emergency release cord. Now you can move the door by hand. Look at the metal rail the trolley rides on. Apply a light household oil or a specific garage door lubricant (not WD-40) to the rail. Also, look at the small gear on the motor unit that engages the trolley—put a dab of grease on those teeth. Reconnect the door and plug the opener back in.
If That Didn't Work
The grinding might be coming from inside the motor unit's gearbox. The Atoms uses a steel-reinforced nylon drive gear. If that nylon gear is starting to wear out and strip, it can make a nasty grinding sound on startup. This is a more involved repair requiring partial disassembly.
Heads Up
Listen carefully. If the grind happens only when the door reverses direction (like at the very top or bottom), that's more likely a limit switch issue. If it's a constant grind the whole time the door moves, stop using it and get it checked—you could be damaging the gear system.