Mine does this too, it sort of creaks / whirs. Assuming there's something caught in the blower. Does your sound go away when you turn it all the way up too? When mine goes up past about 4 blocks it quiets. Someone can advise further (please) but I think we're going to have to disassemble the passenger side lower dash and remove whatever is in the blower or replace it.
2006 BMW 325i | Monacoblau | Beige Interior