Diagnosis
This is almost certainly a restricted airflow problem. Either the filter is filthy, the coils are clogged with dust, or the fan blade itself is gummed up.
How to Fix It
Start with the simple stuff. 1) Clean that foam filter as described earlier—let it dry fully before putting it back. 2) With the unit off and unplugged and the filter removed, peek inside with a flashlight. Look at the aluminum evaporator coils. If they're matted with dust or lint, you can gently vacuum them with a soft brush attachment. Be very careful not to bend the delicate fins. 3) While you're in there, see if you can spot the fan blade. If it's accessible, a quick wipe with a damp cloth can remove dust buildup.
If That Didn't Work
The fan motor itself could be failing. Listen closely—if it's making a grinding or whining noise, the motor bearings might be shot. This requires a parts replacement.
Heads Up
When cleaning coils, never use sharp tools or high-pressure air, which can bend the fins and make airflow worse. A soft brush and a vacuum are your best friends here.