It's possible something has gotten in on the fan and started to build a nest or a glob from mud dobbers. Ifyou go up and pull the shroud off you should be able to see if there is anything on the fan blades.
If that isn't the problem you might want to take the cover ring off the bottom of the unit on the inside and check to see if the bolts that secure it to the roof are loose. If they are, "don't" over tighten them.
They only have to be snug.
2008 Cruiser CF26RK
2013 Ford Eco-Boost
Heavy duty payload - Max tow