Loud humming noise as fridge is running

My Samsung fridge is not yet a year old. It has started this loud humming noise while it is running. it varies how long this noise will go on, sometimes a few minutes other times a half hour. I flip the breaker to make it stop but ultimately it starts again. This happens many times in a day. I have opened the doors to see if the noise is loud inside the fridge and it seems to be coming from behind. There is a protective cover on the back so I can't see what is going on while the noise is present. Any suggestions?
GE refrigerator is about 6 years old. It has a pretty loud hum that stops when you open the door on either the fridge or the freezer side. As soon as the door closes it starts again. Suggestions?