DIAGNOSIS Yeah, this one's common. That's usually the anvil—the part that holds the bit—or the hammer mechanism. A lack of grease or metal shavings in the gear case causes the grinding.
FIX Remove the bit. Look inside the collet (the chuck). See if there's a bunch of black gritty dust or metal flakes. Blow it out thoroughly with compressed air (can of air works). The real fix often requires adding grease. You need to remove the rubber boot around the anvil to access the grease port. Peel it back, and you'll see a ring. Applying a small amount of impact-rated grease (like DeWalt's own) to the exposed ring and anvil can quiet it down significantly.
IF THAT DIDN'T WORK If the noise is a high-pitched whine or squeal from the motor itself, not a grind, that's different. That could be a failing motor bearing, which isn't a user-serviceable part.
HEADS UP Don't use WD-40 or general oil as a substitute for grease here. It'll attract more dust and make things worse. If the grinding is severe and the tool has lost impacting power, the hammer or anvil teeth might be stripped, which needs a repair center.