Just as the frig is about to stop cooling cycle there is loud knocking noise, usually two quick knocks but lately has started to be three knocks. Sometimes but rarely it shuts off with no noise. I have looked all at the fan mounts and what little is found underneath the frig. I have taken a look at the evaporators inside the frig, all seems tight. Frig is still cooling and has been doing this for atleast a year of more. Thanks.