Whirlpool freezer loud clicking sound and going too cold ?

The thermostat on my Whirlpool Easytronic freezer is playing up and rather than being at -18 the tempeature is dropping to -21 and is accompanied by a loud clicking sound. Can't find an instruction book to get any answers. I've tried defrosting it but that hasn't work. Its got to be about 10 years old so its done well but its still immaculate so if anyone knows a solution to the problem please help!! Thanks.