AC noise at idle

[vjharpe]

my 90 240sx makes a loud, sort of grindingnoise in the AC at idle or when the AC loads up. It blows cold when the revs are upand its not loaded down. I replaced thebelt, but its still happening. Someone suggestedI add some oil, as in a system this old the oil"wears out". But I don't want to overfillthe system.

Suggestions?

Thanks