I usually find that when I here a strange noise from a piece of machinery, I later regret ignoring the noise and hoping it goes or went away. I'd pull that turbine down and have a look around at the first opportunity. I'd hate to see a small bearing issue cost you a stator or a blade set or more. Just because you could rebuild the whole machine doesn't mean you should have to. Rich'A Joule saved is a Joule made'