Is this still working ok? I know it's a while since you posted, but we seem to have the same problem on a new convertible only picked up three weeks back. If it's easier, I might just try to fix myself rather than waiting for the BMW Gummi-stuff to be applied...I cured this on my car by squirting a small amount of Halfords window rubber lubricant into the seal around the window pane of the rear quarter windows. Now they are silent and it had been bugging me since day one (18 months).
Apart from that, an excellent drive!:biggrin: