DIAGNOSIS That's almost certainly the oscillating mechanism coming loose. The star-shaped bolt that holds the oscillating mechanism to the motor shaft can work itself free over time, causing metal-on-metal grinding.
FIX Unplug the battery. Look at the front of the tool where the blade attaches. Use the included 5mm Allen key (or a 5mm hex wrench) to tighten the large, central star bolt that's visible in the middle of the oscillating plate. Tighten it clockwise firmly, but don't go full Hulk on it—you just need to re-seat it.
IF THAT DIDN'T WORK If the noise persists after tightening, the internal bearing on the oscillating mechanism might be shot. This usually happens if the tool was run loose for too long. That's a repair for a DeWalt service center.
HEADS UP Get in the habit of checking that star bolt for tightness every few times you change blades. Vibration from use is what loosens it, so it's a recurring maintenance thing, not a one-time fix.