I have a Norcold 821F that is approx. 10 yrs old. It is occasionally noisier than it used to be. The noise is periodic...it is not always on. There is a whine coming out of it that was not there before. Could the cooling fan motor be dying? Any other ideas? Again, I never noticed the noise before, but it is louder than it used to be. Just wondering what I should be looking at/chasing down/troubleshooting.