Ok well I have 2 macbooks in possession. A black one that mooed (my one) and a white one that didn't moo.
After installing the update on each laptop, both fans run constantly, neither moos and both run considerably cooler.
The black macbooks fan, however, runs a fair bit louder than the white one's. Enough to be heard a number of metres away even when idle.
The white macbooks fan can barely be heard, only if you put your head next to the keyboard.
Therefore there must be some hardware issue with the fans that previously had the 'moo'. I wonder if I take both laptops into an Apple shop they will agree with me and replace the fan in the black one?