Your noise isn't normal and can be corrected. BMW is not stepping up to the plate on this one for some reason. I have 54K miles on mine and never had that sound. This isn't my first BMW convertible it is about my 6th. The only time I have heard this sound and it has been in two different BMWs was the hardware around the latch. You have the right to have this convertible not be a PITA but a joy to drive.
I would pursue the lemon law and see if BMW steps up and if not get one that is not causing this issue.
I really enjoy my car, it is quiet it is fast has great fuel mileage. What you have when cold is not representative of the product they produce.
Best of luck...Brad