Age, I'd say. Sound is rated under certain test conditions, placed in certain orientation, etc.
You have always run this drive in the same orientation? I have a couple drives which get louder if I tip them a certain way, so I consider the orientation does have an effect yet the drive might still be fine.
Not to say that noise is never bad, this drive could be failing, certainly.
Just for grins and giggles, is there any discussion in the technical details of the drives rating about orienting the drive a certain way? Once upon a time there was a drive that would suffer bearing damage if operated in a certain position, or so I was told.