Is it making the same noise at all rpms? or is it just at idle, or under load, etc.?
My 240sx did that after changing the alternator on it, but I also changed the belt, and the belt was a half inch too large and wouldn't tension properly. I ditched that belt (that I got from Pep Boys), and got the proper length one from AutoZone, tensioned it up and there was no more noise after that. But it wouldn't make any noise at idle or below about 2,500-3,000 rpms, then it would whine.
In fact, several people asked me if my 240 was supercharged when I was having that issue. I, of course, just told them yes.