Refrigerator coils must be cleaned at least twice a year to keep the motor humming along. A curved brush made for cleaning bottles does an excellent job cleaning around the bends of the coils - do be careful not to puncture them.
Some refrigerator manufacturers place coils in the most awkward places, which makes accessibility to them to clean almost impossible. If the coils run along the bottom of the refrigerator and not easily accessible to clean them by hand, then an air compressor must be rented to blow the dust off the coils. If this is not done the motor will last barely 5 years.
Experience is a great teacher and I learned the hard way. I cleaned the coils I could reach with the bottle brush and vacuum attachment. A few years later, the motor began running continuously and it was necessary to call in a repairman.