DIAGNOSIS Stop using it immediately. This is almost certainly a failing motor bearing or, more likely, the arbor (the shaft the blade mounts to) is damaged. The sparks are from metal-on-metal contact inside.
FIX First, with the battery removed, try to wiggle the blade side-to-side. There should be virtually no play. If it wobbles, the arbor bearings are shot. The fix is a motor/arbor assembly replacement. This isn't a user-serviceable part; the entire motor module needs to be swapped out by a DeWalt service center.
IF THAT DIDN'T WORK If the blade is tight but it still grinds, remove the blade and inspect the arbor flange (the big washer that holds the blade). Look for scoring, burns, or debris stuck to it. Also check that the inner blade washer isn't missing—the blade must be sandwiched between two washers.
HEADS UP This kind of damage can sometimes be caused by using a warped blade, a blade with the wrong bore size (must be 5/8"), or overtightening the blade bolt. Always use the wrench to tighten, but don't use excessive force.