That is the air equalizing pressure between inside and out. When you open the door warmer air enters the frig. When the door is closed that air is quickly cooled by the contents of the frig and the air's volume decreases. (if you warm up air, it expands - if you cool down air, it contracts) The frig has a valve that permits the air pressure to equalize and makes wierd sounds in the process. If the air pressure wasn't equalized, the low pressure on the cool side would hold the door closed and you would not be able to open it. Many older refrigerators and freezers didn't have such valves and forced you to wait 15 seconds or more for the air pressure to equalize enough through various small leaks to permit the door to open.