Diagnosis
That's almost always a foreign object—like a loose screw, plastic piece, or debris—stuck in the blower wheel or hitting the fan. The plastic housing can also develop a crack or clip failure that lets parts vibrate.
How to Fix It
- Unplug the unit and remove the rear air filter for access.
- Shine a flashlight into the exhaust vent and look for any obvious debris. Use needle-nose pliers or tweezers to carefully remove anything you see.
- If nothing is visible, the blower wheel itself might be unbalanced. This requires removing the rear plastic grille (a few screws), which is a more involved disassembly.
If That Didn't Work
The noise could be from the compressor mounting bolts loosening. This is a less common but known issue. You'd need to remove the entire outer casing to check and tighten them, which voids the warranty and involves electrical components.
Heads Up
If the buzzing is rhythmic and coincides with the compressor kicking on/off, it could be a failing compressor. That's a repair-or-replace scenario, not a user fix.