It sounds like you definitely have a leak and lost your refrigerant. As mentioned, you will need to have the leak repaired and the system evacuated to remove any air that might be in the system.
The clicking noise you hear is the compressor clutch engaging and dis-engaging because the low-pressure switch is closing and opening relative to the compressor cycling off and on. When the pressure gets up to about 40psi, the low-pressure switch closes, allowing power to the compressor clutch. Once the compressor is operating, it quickly lowers the suction line pressure below the low-pressure switch setting of about 18psi, which opens the low-pressure switch contacts and removes power to the compressor clutch. With the clutch dis-engaged, the suction line pressure increase to start the cycle over again, and again, and again...
Any reputable A/C repair shop can fix the system, but as mentioned, let them know what you have and make sure the manual is available so they can see how simple the system really is. Chaffing of the hoses and aluminum fittings have caused too many leaks, inspect them regularly as items can shift over time/miles.