Diagnosis
That's typically something loose interfering with the fan blades. It could be a foreign object that got sucked in, or a filter that isn't seated properly.
How to Fix It
Turn off and unplug the unit. Remove the front panel and all filters. Carefully inspect the fan chamber for any debris like paper clips or dust bunnies. Also, ensure all filters are correctly aligned and snapped into place when you reassemble.
If That Didn't Work
If the noise continues after a thorough check, the bearing in the fan motor might be wearing out. For a purifier under warranty, that's a call to support.
Heads Up
Running it with a persistent rattling noise can cause further damage to the fan or motor, so don't just ignore it.