Why is my refrigerator making strange noises?

Several normal sounds can come from your refrigerator: buzzing occurs when the water valve opens to fill the ice maker; pulsating is the fans/compressor adjusting to optimize performance; hissing/rattling is from refrigerant flow or water lines; and sizzling/gurgling is water dripping on the heater during the defrost cycle. Popping is caused by contraction and expansion of inside walls, especially during initial cool-down, and creaking/cracking occurs as ice is ejected from the ice maker mold. (Page 11)