DIAGNOSIS That grinding noise is almost always a foreign object—a loose screw, a piece of plastic packaging, or a thick clump of dust—hitting the fan blades inside the rear compartment.
FIX Unplug the unit. Remove the exhaust hose and look into the rear exhaust port. Use a flashlight and a pair of long-nose pliers or tweezers to carefully extract anything visible stuck in the fan blades. Also, vacuum out the rear grill thoroughly. Be careful not to bend the fan blades.
IF THAT DIDN'T WORK If the noise persists with nothing visible, the fan motor bearings might be failing. This isn't a common user-serviceable part on this model. You'd need to contact Frigidaire for a service technician to replace the fan assembly.
HEADS UP Never stick your fingers or any tool into the fan area while the unit is plugged in, even if it's off. The fan can start unexpectedly if the unit is in auto mode or receives a remote signal.