I have had this in the past and it turned out to be small leaves catching in the fan blades ( how they got in there was a mystery)Remove the heater fan unit from under the dash on the nearside. It is held in place with a small self tapping screw. Twist the unit after removing the screw and it will drop out (bayonet type arrangement).
Hope this helps,
Yes pretty easy.
Open passenger door. Get on your knees outside the car(!) and look up under the dashboard from the passenger footwell. You should see a black plastic cylinder shaped thing. Remove the fixing screw, twist (anticlockwise I think) about 15mm and it should drop out. I would disconnect the electrical connector to the unit first.
Have fun, Mark