Tough call on A/C work if you don't have the tools.
You could check it with those gauges that come with the refrigerant cans if you don't have a gauge set and hoses. It may need refrigerant, but if it does that means you have a leak.
With a rotary vane (which I am pretty sure these are) compressor, the vanes can slap the compression chamber wall when the differential is too high. It usually is because there isn't enough suction pressure to keep the vane properly seated against the wall, so it bounces causing a rattling noise.
A recharge will solve a problem like that. So, check the gas.
If the charge is fine, the compressor may be on its way out; however I'm not willing to write it off just yet. (not that I could without physically seeing the car).
If it has a leak, though... You'll have to take it to a shop. You can't fix a leak and just add gas without drawing a vacuum.
'09 PSM G8 GT- Mods Planned: Stage 1C Livernois, OPG CAI, Kooks Mids, Solo Axle back w/ X-Pipe, GXP Trans Cooler, 2800 Circle-D Stall Converter, Differential (Max 3.45), Suspension mods (Bushings, sway bars, etc)