My 2008 Mini S, Convert (US) has a metalic sound that comes out of the interior fan when the AC is on, no similar noise when the heater or just fan is on. Sounds like a tin can having a spoon lightly rubbed on the inside in circlular motion. I don't understand why a convertible has AC, however, I turned it on to cool down one day with the top up.
Any thoughts?

 

There may a a flapper door that is rubbing, but it isn't normal and should be a waranty issue.