DIAGNOSIS Yeah, that's a classic sign something is loose inside the barrel or, more likely, there's a foreign object caught in the filter or the fan intake.
FIX Immediately turn it off and unplug it. Check the filter first (twist off the base cover). See if any hair, lint, or debris is stuck in there. Clear it out. Next, inspect the attachment you're using. Make sure the long styling barrel is clicked securely into place—a loose fit can cause a vibration rattle. Also, check that the coarse and smooth brushes are fully seated on their cuffs.
IF THAT DIDN'T WORK If the noise is clearly coming from inside the main handle (a grinding or high-pitched whir), the motor bearings might be damaged. This isn't a fix you can do at home. Time to call Dyson.
HEADS UP That rattling is the machine telling you to stop using it until you find the cause. Forcing it to run with something caught inside can kill the motor for good.