It sounds like it is gummed up.
It could be filled with dust/talc if it is from an internal bathroom (i.e. one that has no outside wall, before you blighters all start poking fun with "I thought all bathrooms were internal in this day and age" etc.).
Or there might be something stuck in the blades of the fan.
Switch it off and have a look at the fan blades. You can try to hoover it out if it is dust or remove any obstruction if it is not. Otherwise, you will have to remove it (take the cover off and pull the fan out, having first switched off the mainboard switch) and clean it.