You have a vehicle that is almost 20 years old. There is a VERY good chance that the compressor is dying of old age, even if it's filled to the correct levels with refrigerant and oil.
BE CAREFUL. If you do NOT fix it, the compressor WILL GRENADE. Also, the longer your compressor runs while growling, the greater the chances are that the particles being shed by the accelerated wear of the compressor will spread throughout the AC system, resulting in the need to flush and/or replace more components to restore system function.
1992 XJ -- stock (RIP)
2000 XJ -- lots of basic mods. Sold to fellow forum member. I miss it.
2014 JK -- the project is just starting!