DIAGNOSIS That's almost certainly a problem with the duplex unit—the part that flips pages to print on both sides. A gear inside has likely stripped or something (like a stray staple or torn paper) is jammed in the mechanism.
FIX Power off the printer. Open the rear door. Firmly pull down the duplex unit (the green-ish plastic assembly) until it unlocks and you can remove it. Inspect it for any bits of paper, especially around the rollers and gears inside. Clear anything you find. Also, check the cavity in the printer where the duplex unit sits—shine a light in and look for obstructions. Reinstall the duplex unit by aligning it and pushing it up until it clicks firmly into place.
IF THAT DIDN'T WORK The noise could be coming from the toner cartridge area. Remove all four toner cartridges one by one, give them a gentle shake, and reinstall them, making sure they are fully seated and the latches click down. A failing cartridge drive can cause grinding.
HEADS UP Don't ignore this noise. Continuing to run it with a damaged duplex unit can cause more severe internal gear damage, which is a much more expensive fix. If the noise persists after cleaning, the duplex unit itself may need replacement.