i have the same problem in my r33. but when i start it up, u can vaguely hear the screeching sound. rev it up in neutral, and the screeching noise get worse. i drive down the road for a while in gear and noise continues, but, when i chuck it in neutral the noise goes away. then once i put it in gear again the noise returns.
and sometimes, the noise will be there, the same sh#t as above will happen, then it will go away for a couple of minutes, then come back again.
i took it to my mechanic and asked him to check the belts, which he did, and he said the problem was fixed. but the noise is still there, and happening for longer, only on cold starts though