Water Bubbling -----
Some times described as the sound of a fountain. Primarily heard on refrigerators with a water dispenser. May be the water storage bladder inside the refrigerator section. Usually hidden behind a crisper or shelf. It pre-cools a small amount (one or two glasses) of water so the water dispensed is not warm. Could be caused by an air bubble within the bladder, or the sudden warming of the bladder itself. Sudden warming can cause the water to expand. Sometimes heard when standing with the refrigerator section door open for few minutes.

Water Dripping -----
The defrost system may be operating. If accompanied by hissing, you’re hearing water hitting the defrosting elements. Water dripping sound often heard as the result of water flowing off the cooling coils during defrost, and down the drain tubing. Sometimes accompanied by a gurgling noise. These sounds are normal.

Gurgling --------------
Primarily the sound of refrigerant evaporating (boiling) as it enters the cooling coils. Usually heard just after the refrigerator starts or stops. This is a normal sound for modern refrigerators. In extreme cases may require the addition of a sound-adsorbing pad.

Whistling -------------
Air moves around the interior of many refrigerators even when the refrigerator appears to be off. The air is being moved from a cold area to a warmer area. This eliminates the need to start the compressor simply because one area needs more cooling.