A consistent hum is normal fan noise. A buzzing or rattling indicates a loose component or obstruction.
Turn the unit off and unplug it. Check for external obstructions. Remove the grill cover and inspect the filter. Ensure the True HEPA filter and the pre-filter are correctly seated and not warped. A misaligned filter can vibrate against the housing. Reinstall the grill cover securely, ensuring all clips are engaged.
If that didn't work, the issue may be internal. The fan motor or its mounting could be loose. Also, ensure the unit is on a completely firm, level surface. An uneven floor can amplify vibrations.
Heads up: Do not attempt to open the main unit body. Internal repairs require a qualified technician.