DIAGNOSIS Loose debris inside the housing, almost certainly metal shavings from the plunger mechanism or carbon brush dust. The DCS334B has vents that can let fine particles in.
FIX Clean it out. Remove the battery. Use compressed air and blast it into all the vents—near the trigger, motor housing, and battery port—while holding the tool over a trash can. Tilt and shake the tool to dislodge particles. This is routine maintenance.
IF THAT DIDN'T WORK A more serious rattle could be a detached internal component. If cleaning doesn't stop it and performance is affected (unusual noise or vibration when running), the internal bearing or a motor magnet may have come loose, requiring service.
HEADS UP Some carbon brush dust is normal for this brushed motor tool. If you're cutting a lot of metal, fine metallic dust can be drawn in. Regularly blowing out the tool after heavy use prolongs its life.