Compressor is on the way out, I would not use it until you replace it. Happened to my wife's car a few years ago and new compressor fixed it. Luckily, they are easy to get at on these cars, just gotta purge/service up the AC afterwards since the lines will be open to air. Get the compressor from rockauto and then install yourself or find a good AC shop if you are unable. Like I said you'll need to have it serviced either way unless you have the vac/service kit.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.