i've had this problem.there is a build up of ice behind the fan housing.turn unit off.remove trays.undo and remove 4 screws.let unit defrost.this can be speeded up by use of a hair dryer(not reccomended as water and electricity dont mix well).gently remove fan housing, you will find a big build up of ice.when fully defrosted, re-assemble.turn unit back on.
nick.f
December 2010