If your fridge freezer has a fan inside it to circulate the air it is possible that ice has built up near the fan blades and the blades are now touching that ice. Only suggestion is to turn off the fridge freezer for 24 hours, leaving the doors open. (Put your food in someone fridge freezer for that time). If this stops the noise then that is the cause. Bar removing the interior back of the fridge/freezer and modifying it (don't do it unless you know what you are doing) there is little else you can do other than as stated above evry time it happens.
Regards, Geoff
Geoff Parselle, November 2007