Why is my refrigerator making unusual noises?

Several sounds are normal for this refrigerator: buzzing occurs when the water valve opens for the ice maker, pulsating comes from fans and the compressor adjusting, hissing or rattling is from refrigerant flow or water lines, and sizzling or gurgling occurs during the defrost cycle. Popping sounds come from the contraction and expansion of inside walls, especially during initial cool-down. (Page 11)