DIAGNOSIS: The evaporator fan motor is hitting an obstruction or its bearings are failing. Often, it's just ice buildup on the fan blades.
FIX: Unplug the machine. Remove the rear panel (usually a few Philips screws). Locate the small fan near the compressor. Manually spin the blades and check for ice chunks or a wire touching them. Clear any debris.
IF THAT DIDN'T WORK: The machine might be on an uneven surface, causing the compressor to vibrate loudly. Place a small, non-slip pad (like a silicone trivet) under the unit to dampen vibrations.
HEADS UP: If the noise is a rhythmic grinding that gets worse, stop using it. A failing fan motor can overheat. Continuing to run it risks more costly damage to the compressor.