Diagnosis
That's often a foreign object—like a loose screw, a piece of packaging foam, or a twig—that got sucked into the fan wheel or is rattling around the fan housing. It could also be a fan blade that's cracked or come loose.
How to Fix It
Power off and unplug. Remove the air filter and use a flashlight to look deep into the intake vent for the indoor fan. See anything? Try using needle-nose pliers or a coat hanger hook to carefully fish it out. Also, remove the rear exterior grille (usually a few screws) to check the exhaust fan side for debris like leaves.
If That Didn't Work
The noise might be internal. The compressor mounts can loosen over time, or the blower wheel on the motor shaft might have slipped. That's a more serious disassembly job for someone comfortable with taking the cabinet apart.
Heads Up
Never stick your fingers or any tool into the fan area while the unit is plugged in, even if it's off. Those fan blades are sharp and can spin unexpectedly.