If the charge is low the compressor will only operate briefly before it shuts off. When the charge is very low, the compressor will not kick on. But the grinding noise is bad news. A good compressor will make a light pumping sound, but should never sound like grinding. If it is making a grinding noise, do not operate it. The grinding is probably the compressor self destructing. Remove the belt from the compressor and see how it feels when you turn the clutch by hand. If it is hard to turn or grunchy - bad news. A good one will turn fairly easily with a noticeable slight pumping resistance. If it is going bad, the little bits of metal from the grinding you feel can circulate through the system requiring a flush to make sure the metal bits don't destroy the new compressor you install. If you find bits of metal in the discharge manifold of the compressor, you will need a thorough flush or replacement of accumulator, lines, orifice tube and condenser. Some like to add a filter as extra protection for a new compressor after a flush.