Miele refrigerator Model KFN 8993 SDE (I believe)

Question

I have a free standing Miele fridge that won't stop beeping.  It has an error code of 'F4' but I don't have an operating manuel for it.  It doesn't have frost or buildup.  Nothing unusual has happened.  The non stop beeping is driving me a little crackers and I really need your help.  I've tried altering the temp gauge to see if that stops it and sometimes it will for a while.  It starts it's beeping usually in the early evening and will continue until dawn (very antisocial)