DIAGNOSIS A rattling noise at high speeds is typically a loose filter or foreign object in the fan assembly.
FIX Power off and unplug the unit. Remove the front panel and both filters. Check for and remove any debris like plastic wrapping or tags inside the filter chamber. Reinstall the filters, ensuring the True HEPA filter is fully seated at the back.
IF THAT DIDN'T WORK The noise may be a loose fan blade. This requires disassembly not recommended for users. Contact Coway support.
HEADS UP A consistent low hum is normal, especially on speed 3 or 4. A new rattling or grinding sound is not.