Diagnosis
That's not normal operation noise. A steady hum is fine, but a rattle or buzz is usually the front plastic grille vibrating against the main cabinet. The clips can get loose over time.
How to Fix It
Turn the unit off. Firmly press along the edges of the front grille with the palms of your hands, top, bottom, and sides. You're trying to snap all the plastic clips back into their slots. Often, you'll hear a few clicks as it reseats. Turn it back on—the rattle should be gone.
If That Didn't Work
The noise could be coming from inside. With the unit unplugged, remove the grille (it pulls straight off). Check if the fan blade has any debris (like a leaf or twist-tie) stuck in it, or if it's loose on the motor shaft. Also, make sure the compressor mounting bolts (the heavy part in the back) are all tight.
Heads Up
Never stick your fingers near the fan or internal components while the unit is plugged in. If the internal fan blade is cracked or damaged, it will be unbalanced and noisy—that requires a replacement part.