DIAGNOSIS That roaring or vibrating lawnmower sound is almost always the exhaust hose. When the high-speed fan kicks in, it can cause the flexible hose to vibrate against a wall, window sill, or itself, or the internal spiral wire can resonate. A kink in the hose will also make the fan work harder and louder.
FIX First, check the hose path. 1) Make sure it's as straight and short as possible from the unit to the window—no sharp bends or kinks. 2) Where the hose touches anything (wall, floor, curtain), put a towel or foam pad between them to dampen vibrations. 3) Gently twist the hose along its length; sometimes just changing the position of the internal coil stops the resonance.
IF THAT DIDN'T WORK The noise could be coming from inside. Ensure the unit is on a firm, level floor. Carpet can absorb sound; a wobbly stand or uneven floor can transfer compressor vibrations. Try placing it on a rigid board or mat designed for appliances.
HEADS UP Some operational noise is normal—you're hearing a compressor and two fans in a small cabinet. But a consistent metallic grinding or banging isn't normal. That could mean a fan blade is hitting something or a component is loose, and you should contact support.