There is a noticeable noise coming from the engine compartment when the A/C is selected.
A compressor noise is often difficult to locate. When the compressor is operating, a loose compressor mount or bracket may vibrate, giving the impression that the compressor itself is somehow defective. This type of noise usually stops when the compressor is disengaged.
A/C compressor off.
If the system did not have any refrigerant in it, then the low pressure devices would prevent the A/C compressor from running.