Diagnosis
Metal-on-metal contact. The blade is likely hitting the debris shield or a foreign object is lodged in the blade housing.
How to Fix It
Unplug the mower. Tip it on its side. Manually rotate the blade. Listen and look for contact points. Clear any wrapped grass, wire, or stones from the blade shaft and under the deck. Check that the debris shield (the plastic guard around the blade) is not cracked or warped into the blade path. Ensure the blade bolt is tight.
If That Didn't Work
The motor shaft bearings may be failing. This produces a consistent grinding from the motor housing itself, not just from the deck area. This requires motor service.
Heads Up
Never operate the mower without the debris shield properly installed. It is a critical safety component.