That doesn't sound normal to me. Granted I've only had my 335i vert for a month or so, but it's an '11 and is as quiet as a church mouse. That rattle certainly isn't normal/acceptable if it's coming from the top. Have you been able to locate which quadrant it's coming from?
I defer to Kat, whose infinite and intimate knowledge of the top seems unending, but it sounds to me like a joint has too much play or there's a deteriorated seal at a pin/lock.