I have a 97 Infiniti j30 that makes noise every time i run the AC. How do i know if its just low on lubricant or if i need to replace the ac compressor. Also how hard is it to replace it if i were to do it myself?

It's not that differcult to change, but does take alot of time. You're going to have to remove the bottom cover, remove the fan, those things that look like hoses at the top of the engine, do keep in mind that time is a factor, just start early on saturday morning. Oh you will have to refill your freon. I really don't know how to determine if you need more lubricant. My thoughts are that if your a/c has been working successfully all this time then changes are that you don't need lubricant. Especially, if you haven't had any leaks.