After copying my data from the 15inch MBP to my new MBA, I found that the fan runs at full speed irrespective of what processes are running. So I decided to reset the System Management Controller (SMC) and this actually helped.
What you must do is switch off MBA while it is still plugged in to the power adapter and press
ctrl+option+shift and the power button (top left corner) just once. The light on the power adapter will
turn into green and after couple of seconds back to orange. Now switch the machine on.
Further info is here
