Refrigerators can produce a variety of noises, not all of them bad, and just because you can hear a regular pulsating sound doesn't necessarily mean that part of the system is failing. It might be the normal operating noise level of the evaporator or compressor. But it's also possible that the pulsating sound is not normal and is the harbinger of a more serious problem.
The compressor is a black, football-sized metal object on the back of the refrigerator. As the heart of the refrigeration system, it compresses the refrigerant gas powering the cooling process. A pulsating sound might be part of normal operation or it could be a signal of imminent mechanical failure. With compressors being costly to repair, if your refrigerator is out of warranty, it may be less expensive to buy another appliance. If the warranty is still valid, call a repairman before it expires.
Another possible trouble area is the evaporator, which can produce several different noises, including a pulsation, for various reasons. Located in the body of the refrigerator, the evaporator is responsible for absorbing the heat from inside the cooling compartment. Visually, it resembles a radiator, complete with a fan to disperse heat. A worn bearing or other problem with the fan motor could cause it to give off an abnormal pulsating sound as it rotates.
On its journey from the condenser to the evaporator, refrigerant is transformed under great pressure from a liquid into a gas. If the system is low on refrigerant and in need of charging, the inadequate level inside the tubes could cause an assortment of knocks, pings or pulses. It's difficult to detect a refrigerant leak since it is a colorless, odorless gas, but an accumulation of oil near joints or connectors is a clue. It might be necessary to call a fridge technician to verify this is the problem.
The pulsating sound you hear from your refrigerator could also be a simple case of a loose screw. The compressor, condenser and evaporator are all attached to the appliance's body with screws. Over time, one or more of these screws could work their way loose, causing a vibration as refrigerant is cycled. One of the first troubleshooting measures should be checking that all screws and bolts are tight.