Why do refrigerators hum?
Well, fluorocarbon coolant cools your food by coming to a boil in coils hidden in your fridge.
How is this possible?
When you boil water on the stove, you supply heat energy by turning on the burner. In your fridge, liquid coolant boils at very chilly temperatures, but it still requires heat energy. The coolant absorbs this heat energy from the air inside the fridge. By the time all the coolant has boiled and evaporated into gas, the fridge has reached Arctic temperatures.
What happens when the temperature starts to rise again? In order to repeat this chilling performance, the gas coolant has to release heat energy and change back to liquid state. To release heat, the coolant is forced into a second set of coils outside the fridge, in back. Sometimes these coils feel pretty warm.
How does the gas coolant that just chilled your fridge make the coils warm?
The secret's in the refrigerator's hum. That humming is a pump that compresses and warms the gas while forcing it into the coils. It's like pumping up a bicycle tire; the tire feels warm because you've compressed the air inside.
Through compression, the gas coolant in the coils has become warmer than room temperature and can now release its heat energy to the air. As it releases heat, the gas cools and condenses back to liquid state. When your fridge hums, it's releasing heat energy and changing fluorocarbon back to liquid state. Then it can cool your food by coming to a boil once more.