Popping sound from refrigerator

Question

I have a 6-year-old Kenmore top-mount refrigerator that four months ago started giving off a loud popping sound about every 30 minutes. The refrigerator works fine, but I'm wondering if something has gone wrong. It seems unlikely that this sound would all of a sudden come from the expansion and contraction of the inside walls of the refrigerator. We haven't cleaned the coils on the refrigerator because of the diffulty in pulling the unit away from its spot in the kitchen. Any advice?