DIAGNOSIS: The unit isn't level, or the compressor's vibration is transferring directly to an uneven floor surface (like thin carpet over hard floor).
FIX: First, check that all four casters/wheels are firmly touching the floor. Adjust by placing thin, solid shims (like folded cardboard) under any wobbly corner. Do not place on a thick, soft rug.
IF THAT DIDN'T WORK: The internal compressor is mounted on rubber isolators, but on some surfaces, you need extra damping. Place the entire unit on a rigid, level platform like a piece of plywood or a low-pile anti-vibration mat designed for appliances.
HEADS UP: This walking is a safety issue—it can strain the power cord, disconnect the exhaust hose, or tip over. Secure it. Also, ensure the exhaust hose isn't taut, pulling the unit, and that the window bracket is installed securely, not transmitting vibration from outside.