Water Fill Line
If your refrigerator has an automatic ice maker inside the freezer compartment, then the humming noise you are hearing could be a normal part of operation. This is especially true if the noise only happens occasionally or at regular intervals. Ice makers have a water fill line that comes in from behind the refrigerator and fills up the cube molds for freezing. When the ice maker calls for more water after dumping out a batch of cubes, a pump kicks on and sucks the water through the hose and fills the ice maker up again. This pump's action often results in a humming noise you can hear in another room.
Self Defrost
Depending on your model, you may have a refrigerator with a self-defrosting feature. This is typical on new refrigerators by GE or other companies. When the unit defrosts to keep ice buildup from hindering cooling, the melting ice produces drops of water that drips on the defrost heaters. The resulting sound can be interpreted as a hum, hiss, sizzle or other noise that could explain the odd things you are hearing. This will only happen a few times per day. It is also a completely normal sound and it will not damage your refrigerator.
Compressor
The humming coming from your refrigerator could be the compressor doing its job. The way that refrigerators create the cool air inside is by drawing cool air over cold coils with a fan. But the way the coils get cold is the more interesting part. A compressor is a pump that compresses the gas in the system that causes it to warm up. When the gas passes through an expansion chamber and expands rapidly, it quickly cools down causing the icy cold coils needed for refrigeration. The hum that often comes from the refrigerator is the normal sound of the compressor pumping the air and compressing it during this process.
Fans
In order to pull the air over the chilled coils, the refrigerator needs an evaporator fan. This fan spins, which causes a whirring or humming noise. Occasionally, a refrigerator will make odd humming noises because the fan constantly speeds up and slows down. This is not normal and could be caused by a bad speed controller on the main board of the refrigerator. This will require repairs, but if it only happens occasionally then it is likely nothing to worry about.