Probably the fan is on the way out but honestly that post didn't make much sense even on the fifth re-read. What do you mean by when the case is off? do you mean that you've taken off a sidepanel or something? If that is the case, pun not intended, that removing bits of your case reduces the noise you should be looking for anything that rattles. Just get your head in there with your ear up close to everything, don't be afraid to apply pressure to panels on the case to see if that reduces the noise.