The Amana refrigerator is less than a year old. From day one it makes a soft knocking sound, like somebody knocking at the door. It is not continuous, just off and on. Sometimes it happens after opening the refrigerator or freezer to take something out but there are times when neither have been opened and the appliance makes a knocking sound. The appliance seems to be functioning fine but can somebody explain the knocking sound? This sound can't be normal, right? Do I need to make a service call?