DIAGNOSIS That's the sound of the blade hitting something it shouldn't. Either the blade is bent, loose, or there's a foreign object wedged between the blade and the deck.
FIX Disconnect the battery. Tip the mower on its side (air filter side up). Manually try to spin the blade. It should turn freely and not wobble. If it hits the deck, look for a bent blade or a stick/rock jammed in there. To check for looseness, try to wiggle the blade—it should be torqued down tight on the spindle. A common issue is the blade adapter (the star-shaped piece) coming loose from the motor shaft.
IF THAT DIDN'T WORK The noise could be from the gearbox that drives the blade, which is a separate unit. If the blade spins freely but the grinding noise comes from the center housing when you try to start it, the internal gears might be damaged. That's a warranty/service issue.
HEADS UP Always remove the battery before inspecting the blade. Even with the key out, capacitors in the system can hold a charge. Never grab the blade to try and stop it—let the system fully power down.