Your Mac has sensors that respond to temperature changes. It may be normal to hear the fan running in the background during intensive tasks or having multiple tasks open. The rushing-air sound is a normal part of the cooling process. 

If you notice a fan noise louder than usual, you may first want to make sure that your fan vents are not blocked (Macbook Air). Resting your device on uneven surfaces like your lap, couch, bed or pillows - may make the fan run more. Using your device on a hard surface like a desk is best for temperature control. 

If the fans in your Mac run faster than usual without running intensive tasks and being properly ventilated, you may need to reset your SMC.

Before doing this on a a Macbook Air, be sure to try these steps first:

  • Hold Option-Command-Escape and force quit any unresponsive applications
  • Place your Mac in Sleep from the Apple Menu. Wake the mac after it has been put to sleep.
  • Restart your Mac by choosing Restart from the Apple menu
  • Shut down your Mac by choosing Shut down from the Apple menu.
  • If your Mac is not responding, hold the power button for 10 seconds - you will lose data from any unsaved applications.

If all of those steps fail:

  • Power off the Macbook Air completely
  • Plug it into a power source, making sure it is not fully charged.
  • Hold Shift-Control-Option and the power button simultaneously, hold these for 10 seconds
You will notice your charger light switch from Amber to Green and back to Amber

  • Power your Mac back on by pressing the power button

Your SMC has been reset.

If your fans are running faster than normal on a Mac desktop, repeat the steps from above before performing an SMC reset.
To perform an SMC reset on your desktop:

  • Power off the desktop
  • Unplug the power cord from the back
  • Hold the power button for 10-20 seconds
  • Plug the power cord back in
  • Power your Mac back on

Your SMC has been reset.