Jim, we had a steel-on-steel clanging noise issue on Sun (Deck 5) during the last couple of weeks of the WC earlier this year. It took days for the staff to identify it, but our dinner companions (architect and civil engineer) one evening immediately recognised our description as a faulty fire damper in the funnel. Some action was taken to minimise the noise, but it did continue. If it's the same source, you are correct in that it's very difficult to sleep through as it is irregular. Hope they sort it out for you.