I just had the same problem. It means the freezer isn't cold enough.
Is the door shut and sealing properly? Does it create a bit of suction when you shut it after a few seconds? It should for a while.
Check that and turn the thermostat to max for a while and it may get down to the right temperature. I've just done that and after 30 mins it has stopped beeping.
If that doesn't stop it on yours, I suggest you empty it, turn it off for a day, manually defrost it and clean it then turn it on and once it has cooled to the right temperature, hopefully it will stop beeping. That's what I'm going to try next if the beep comes back.
Alex
September 2008