imac late 2012 : fan noisy at startup

Hi,

I'm using imac 21.5 late 2013 with 10.9.1.

Each time, imac start or wake up from hibernate, fan increase speed (istats menu says 4000rpm instead 1400 rpm normally) and become noisy. After some minutes, speed decrease and return to normal. Then fan stay at normal speed for hours of works and all is fine.

Any idea the reason of this anormal speed at startup ??

Thank you for your help.