What ambient temperature do you have? Mine makes noise almost all the time but i notice when there's even a little bit of cool weather the fan doesnt make noise or just takes longer to start making noise. It even went to making no noise after making noise for a while recently which has never happened. That was during some nice cool weather (though with rain ).
It could be a firmware update. I thought that was my problem at first also till i realized that when i got it the temperature was cool and it was now in summer that i was having the problem.
If you really mind it and cant solve the problem you could find a way to get the fan quieter even when running at a high speed.