My a/c on my 1999 gmc sierra blows out air but not cold, also when i turn a/c on it makes an odd sound, but only when i turn the a/c on, not when the fan is blowing

What kind of a sound do you hear when you turn the a/c on? Where is the sound coming from? It sounds like your a/c compressor is going bad. Compressors a lot of the time will make a grinding or growling noise that you can hear from the engine compartment and it will only happen when you turn the a/c on.

Regards
~Brandon

Could be your compressor clutch is slipping. Get it running and look at the clutch. Likely your low on freon.