Yeah, this one's common. The noise is almost always something loose inside the unit or an obstruction hitting the fan.
- Turn the unit off and unplug it.
- Check the grill cover and the exterior for anything that might have fallen in.
- Open it up and check the filter. A filter that's not seated correctly or has come apart can cause a rattle. Make sure the pre-filter and HEPA filter are properly aligned and locked in place.
- Gently spin the fan blade by hand (with the unit unplugged) to feel for any grinding or see if it's hitting something.
If that didn't work, the internal fan motor bearings might be wearing out. A consistent grinding or buzzing from inside the motor housing points to a mechanical failure.
Heads up: Never run the purifier with a loose filter or foreign object inside. It can damage the fan motor permanently.