Diagnosis
The noise is likely from the evaporator fan motor contacting ice buildup, or the compressor mounting bolts are loose. A grinding noise points to fan blade obstruction. A vibrating hum points to compressor or tubing resonance.
How to Fix It
For grinding: This indicates a defrost issue. Follow the steps for evaporator fan obstruction (first Q&A). Clear all frost from the fan path. For vibrating: Unplug the unit. Pull it out from the wall. Check the black access panel at the rear bottom. Remove it. Inspect the compressor mounting bolts. Tighten any that are loose. Ensure refrigerant lines are not touching the cabinet wall; gently bend them to provide a 1/2-inch gap.
If That Didn't Work
The condenser fan motor, located near the compressor, may have a failed bearing. With the unit plugged in and the back panel off, listen for the noise source. A failing condenser fan motor will produce a loud hum or rattle.
Heads Up
When checking the compressor area, do not touch any electrical components. The capacitor can hold a charge even when unplugged. Only perform mechanical adjustments like tightening bolts.