Diagnosis
A grinding noise is typically a foreign object in the fan blades or a failing fan motor bearing. A rattling noise often points to a loose component or the exhaust hose vibrating against the window bracket.
How to Fix It
Unplug the unit. First, check the exhaust hose assembly. Ensure both ends are snapped securely into the unit and window vent. Tighten any adjustable window brackets. Open the front grille and remove the air filter. Visually inspect the interior fan for debris like leaves or plastic. Use a flashlight and long tweezers to remove any obstruction.
If That Didn't Work
The noise is likely internal. The blower wheel may have come unseated from its shaft or the compressor mounting bolts may be loose. This requires disassembly. Check if the noise changes between fan speeds. A consistent grind at all speeds points to the motor bearing. This is a repair for a qualified technician.
Heads Up
Do not operate the unit with a persistent grinding noise. It can cause secondary damage to the fan blade or motor, turning a simple fix into a major repair.