The sound isn't usually too bad but shouldn't be ignored because it can cause other problems to the system. If the unit is working fine then most likely it is just a noisy transformer. Some transformers are louder than others and they all hum to some degree. This is usually refrigerant-flow related, such as a check valve. Not always a problem.
Some units have noisy check valves or metering devices.
If the sound gets louder or annoying and the air filter has been checked then a service call should be placed. This is also only found on systems with sheet metal ductwork and it is usually the return trunk. This could be as sign of undersized duct, a clogged filter, closed vents or flimsy duct requiring bracing. If you hear an unusual rumbling sound associated with combustion from your gas or oil-fired appliance, call for service immediately. This indicates what could be a serious problem with the burner portion of your appliance.