Refrigerator?

My hoover contour HE365UF has started to make a noise/vibration that seems to be coming from the rear.. I also notice an ice build up but not sure if this is normal as it does not appear to be too extreme - when looking in the front on the freezer section, the compressor pump seems to be running continually. I will turn off and defrost but am assuming i will need to replace the defrost switch and or thermostat switch.are they an easy replace ?.. thank you